WIth the new expansion out, there are a lot of mechanics that need to be understood, as well as many smaller intricacies that might have profound impact on your game. To help people a bit, here is a little guide about the one thing I know how to do in Stellaris: Playing a Fanatic Pacifist empire. Many of the things here apply to other play stiles as well, but this is the one I have nailed down, so I feel comfortable enough to actually write a guide.
The setup: Terkardian Utopia
For this exercise I will use an optimized, min/max'ed version of the empire I am playing in the dev clash, the Terkardian Utopia. I usually play on medium sized maps with maxed out player numbers, since I like to see what funny random empire designs the game will come up with. This is definitely favorable for this play style. However, the general idea should translate to any type of map or setup.
Ethics: Fanatic Pacifist + Xenophobe.
Fanatic pacifist is a very potent, all-around useful ethic. The extra admin cap will give you a lot of extra breathing space in the early game, in some cases allowing you to reach the first admin techs without exceeding your cap. In the long run, it is equal to ~10% lower unity cost and ~6.6% lower tech cost. On top of that, the stability bonus will give you ~5% better job output and can compensate for lack of amenities and overcrowding to a certain extend.
Xenophobe has gotten even better for Inward Perfectionists in Megacorp. The reduced claim cost is now replaced by extra pop growth, which is great news for an empire that doesn't care about waging wars. The reduced Star Base cost is useful early on, allowing you to push for rapid expansion if the map offers favorable conditions for it.
A side effect of mixing pacifist and xenophobe ethics is that the game will spawn the Isolationist xenophobe faction, which synergizes very well with this play stile. This means you'll have two factions that have very similar intentions that you'll cater to by just playing the game.
Civics: Inward Perfection + Agrarian Idyll
Inward Perfection is really an ideal pick for us, since we do not plan to wage any wars. We get a whopping 20% bonus on pop growth (which is equal to 4 trait points now), a 20% bonus on Unity generation and a 5% bonus on happiness (which translates to another ~2% output bonus via stability). The fact that we will be limited in diplomatic matters doesn't really matter all that much to us, since we just want to do our own thing anyway, and the AI is very unlikely to join a Federation with Defensive Wars policy in the first place. This civic replaces the Diplomacy Tradition which the very potent Adaptability traditions, which has some very potent benefits for planetary development (including an edict that can create an extra basic deposit creating feature per planet).
Please note that this civic can no longer be removed or replaced after game start, so the old strategy of playing isolationist early and then steamrolling the galaxy late game is now a bit less powerful. Anyway, we are not here to do that. We are proper pacifists.
Agrarian Idyll might not look that great at first glance, but it is a radical and powerful game changer. Back when the old version was changed during the dev clash I was somewhat skeptical (I mean, the previous iteration was easily amongst the most powerful civics in the entire game), but after playing a few games, I have come to really appreciate its power. You lose 1 housing on cities, but gain 1 housing on Generator, Mine and Farm districts. Later down the line, you have access to a tech that adds an extra housing to farms, for +4 housing total.
So why is this good?
Well, the way it generates housing is more or less ideal for the new building slot progression. You build 2 districts to get 6 housing, 4 pop jobs and 1 building slot that the other 2 pops can use however you see fit. This makes it an ideal pick for beginners, since you can completely ignore cities and just focus on whatever you need right now. On top of that you can dedicate ALL of the planet size to resource producing districts. Given that raw resource production is a very limiting factor until you get Ring Worlds (and in the case of M even after getting that), not having to waste any of that potential production by building cities is great!
The price for this is a slightly lower max amount of pops that can inhabit a planet. However, given that the amount of building slots caps out at 75 (or 70 with the Adaptability traditions) means that this lower cap isn't really all that relevant. A regular size 15 planet with 5+5+5 deposits will provide you with 15+15+20=50 housing in the later stage of the game. Now add the extra housing from the main colony building and housing usage reduction effects from Traditions, Gene Perks (etc.) and you'll end up with about enough to unlock all slots anyway. Also, the max population cap of a planet is only relevant in the late game, and by that point you should already have Habitats and Ring Worlds to offload the excess population to.
Oh and: On top of that, all farmers will provide 2 amenities, which is equal to a ~0.25% resource yield bonus on the planet. This allows you to mostly ignore amenity buildings abd instead use building slots for other, more powerful options.
All that being said, there is one distinct drawback to Agrarian Idyll: You do rely on the mercy of random deposits. If you find several size 18 planets with something like 3/5/3 deposits, that is bad news. It is a risk you'll have to take with this civic, but after several games I think the average deposit roll on a planet is pretty good. You will probably end up with one or two duds per game, but you'll manage. Plus: The adaptability traditions allow you to add an extra (basic) planet feature via a special decision, which is on average worth 2 deposits.
Authority: Democracy
This is mostly personal choice. I like democracy because it fits my vision of a harmonious, utopian society. The mandates are also pretty nice, and since you will build a lot of resource districts, they should be easy to fulfill. There is a tendency for the Egalitarian faction to pop up (especially if you colonize a low-habitability planet early on), so if you are really into min/maxing, I guess Oligarchy would be the way to go.
Species Traits: Industrious, Rapid Breeders, Solitary, Slow Learners
Truth be told, I am actually not 100% sure about the ideal choices here.
Industrious is definitely a must-have, since Minerals will be the most limiting factor of a tall empire, unless you are lucky enough to stumble upon a Black Hole (to build a Matter Decompressor). Even then, until you reach that point in the game, you will be very happy for every extra bit of M you can get from your miners.
Rapid Breeders is costly, but growth is one of the most limiting factors for this empire. You could pick another 2 point yield related trait instead, but none seem all that necessary: We will build plenty of farms, so we should have enough F to run constant Nutrition Plenitude. We will build plenty of Generators, so Energy generation shouldn't be an issue. Trade Value is not something that matters in the early game, and by the time it does matter, we can just use gene modding. I honestly think this is a "pick what you prefer most" situation.
Solitary might seem contradictory. Wait, don't we already have a lower population maximum on our planets because of Agrarian Idyll? Yes, but during the early game the resource districts will actually provide more housing than necessary. Once our deposits are used up and our planets start to cap out we can Gene Mod the trait away or even replace it with Fertile.
Slow Learners is again sort of a personal choice. I think of everything left, it is probably along the least disruptive traits. Among the other old popular picks, Sedentary is quite bad in the early game, since you will have a lot of migration from your capital to the new colonies, and it wastes a lot of growth when you need it the most. Weak isn't awful, but I'd rather not take the resource production penalty. Again, feel free to take the ones that you think fit your play style best/impede it the least.
Planet type: Ocean
Because I love ocean worlds. Also, iirc, wet planets have higher chances for F deposits, which is a good synergy effect with Agrarian Idyll.
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Early Game: Expand and Settle
We send off our science ship to survey nearby systems to find our two guaranteed planets. Meanwhile, start building a Mining station on our guaranteed M deposit. Once we have enough Minerals, we build a Mining district on our home world. The first building we want to build is an Alloy factory, although depending on shape and number of habitable planets found, we can also go with a science lab instead. I'd say the Alloy plant is in general a safer option, since it also allows you to build a quick defensive star base or some extra ships if things get really hot early on.
Once you find a colonizable planet colonize it asap. I'd say start with ocean types if possible, since this will reduce the chance for an Egalitarian faction popup and reduces the upkeep drain on your economy during the initial growth phase. Once your colony buildings are up, you'll have to wait. In the meantime, slowly add more districts. Be wary that due to our species Industrious trait Miner jobs will take priority over everything else, so if you build a new M district, it will "suck away" pops from their current employment. I'd recommend to focus the first colony mostly on mining and following ones on a mix fo E and F, with some sprinkled in M districts when your economy is in the green for E & F.
As for our tradition order, I recommend to go with Expansion, right hand side first. The extra pops on colonies and the extra growth will help a lot to get these colonies up and running, and the admin cap on the left hand side is great to delay the tech and unity cost increase.
The first building in every colony should be a Monument. The extra Unity production will greatly help to speed up Tradition unlocks and the extra Social research is useful to get to the admin cap and blocker techs. The second building should be a science lab, so we can get our science progress going. Having that many CG consuming jobs will mean that the capital will be busy building CG factories in most of its own slots at that point and you might actually run out of enough slots there. If that is the case, dedicate the #3 slot of the colony to a CG factory. Otherwise, go with another lab.
In terms of tech, you we want to focus on everything that increases resource job and scientist yields. Other important techs are Fusion Power (since we need to get to Zero-Point for Master Builders), Planetary Unification (which leads to Adaptive Bureaucracy, which is needed for Colonial Bureaucracy, the first admin cap tech), all the resource boosting buildings (we ultimately want all of them on every planet) and research speed boosting AI. Get blocker removal techs as needed for your smallest / most blocked planets. The Starbase techs are also vital to get to Habitats.
When we are done with the Expansion Traditions, we go for Adaptability. This will give us less housing use (which will offset our gene trait), less F upkeep (particularly good when you have lots of low habitability planets), a +10% habitability boost (again, less upkeep everywhere) and (get ready) a free building slot unlock on every planet. Yeah, it is a darn good tradition tree. Oh yes, and the finisher will give you the option to create a new planetary feature for 500 E and 25 Influence (once per planet), which is usually worth 2 deposits. After that I'd recommend to go with Exploration to improve efficiency of labs and reduce the strain researchers put on the economy. Prosperity is demanded by the Pacifist faction, but rather weak, since two of its traditions affect cities, which we only use on planets where we run out of deposits. If you want to pick it over Harmony or not is up to you and sorta depends on your economic situation.
As for Ascension Perks, I'd say - leave them empty for now. I am a big fan of Imperial Prerogative, but that requires a picked perk first, so we will put Voidborn in slot #1. However, this is not something set in stone. If you have a lot of small planets with lots of deposits, Mastery of Nature can be a better first perk choice.
After the first few decades we should have a decently sized empire with around 20 systems and 6 planets or so, which is all we'll need to get our Habitats and Ring Worlds. You can go for more (and certainly should if you find good systems or planets), just keep in mind that you will exceed your admin cap a lot earlier, which makes the mid game slightly slower (at the benefit of a stronger late game). Try to expand to choke points so you can minimize the amount of Star Bases used for defence. If you find an AI empire, feel free to gift it some excess resources via monthly trade to bump their opinion of you. Military buildup is very costly in Megacorp, so avoiding wars will greatly benefit you. If push comes to shove, build a Starbase at the border to your potential enemy and add some guns to it. Silver lining: It will provide some trade protection that way.
Speaking of trade, you will want to make your capital star base the main trading hub. Each bit of TV it collects is a bit of TV that you don't have to protect from pirates. Given that most planets have very low TV generation in the early game, it is not entirely necessary to pick up every little bit of TV. Border defence comes first, trade collection second. Again, try to centralize collection star bases so they can affect as many systems as possible. Use corvettes in areas that are not covered by defensive star bases to suppress piracy as needed. Given that the initial TV generation is low and that your empires should be rather compact, a handful of corvettes should be able to cover all the vital systems. In terms of choosing your Trade Policy, I do like the Consumer Good Benefits setting, but you can argue that as long as the buying price on the market isn't too high, you can stick with Wealth and just collect more E to spend it on market trading.
So, one early game question remains: To use robots or to not use robots? I'd say: Nay. You have high inherent pop growth that is good enough to allow your empire to grow roughly in sync with tech advancement. In addition, since we run Agrarian Idyll we have a tendency to overproduce F anyway, which means shifting pop upkeep cost from F to E is not a particularly efficient thing. Consider it if you have tons of low habitability planets, but otherwise - you'll be fine.
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Mid Game: Develop and Tech
Once our colonies start reaching enough pops for the second colony building upgrade, we have reached the mid game. Build one instance of each resource boosting building (Nexus, Mineral Processing, Food Processing) on every system. If you have researched it, put a Science institute in the slot that comes with the 40 pop colony upgrade. If you have any special features on your planets that generate refined resources, you can start to collect them - small planets or planets with low deposits might at this point start to get into situations where you need to upgrade a building or two to get enough jobs for your pops.
Regarding the economy, try to run a modest surplus of everything, but not too much. You'll probably want something like +30E, +75M, +30F to pay for constant planet development, blocker clearing and edict refreshing. If you get far more than that, consider building a few more labs - or an alloy plant or two, given that we will soon reach the point when we get to unlock Habitats. Excess influence can be used on yield boosting edicts. If your economy is doing fine, you should switch your living standard to Social Welfare - it provides a quite modest bonus to happiness and stability and will allow unemployed pops to generate a bit of Unity until they get proper job assignments.
This should also guarantee that you won't see any crime on your planets. Well, unless you are unlucky and you have a Crime Syndicate nearby. As a Fanatic Pacifist, you cannot expel their branch offices via war and since you can only close down one every 20 years (silly hidden cooldown modifiers), so there is nothing you can do against these guys. Your best option is to strike a deal with the crime lords on affected planets. That will cause crime to skyrocket, but you will gain +10% stability (which is REALLY good) and the deal will prevent any crime events outside the initial establishment of a criminal underground. Said event will apply a modiifer that can suck up to 4 pops into criminal jobs, but the +10% stab from the deal is more than enough to compensate you for that. It's actually not that bad.
Around year 60 or so, we should be able to get the Star Fort technology to unlock Voidborn. This marks an important junction where the game will diverge from what you are used to from Utopia/Apocalypse: Habitats are no longer something you spam everywhere - they have significant alloy upkeep cost, more than a Titan class ship! They are a tool to be used in moderation to generate more building slots and extra trade value. For now, 3-4 should suffice. If you have plenty of M deposits (or access to a Black Hole), I'd recommend to aim for a setup with 5 habitat districts + 3 trade districts, otherwise go with a 4-4 split instead. Once they are up and running with some slots unlocked, you can start building the special resource refinement buildings on them, as well as a few CG and Alloy plants. Use these resource to upgrade the resource booster buildings and buildings on your planets that are running out of pop jobs from new deposit districts.
Midgame key techs to look out for are Zero Point Power (for Master Builders), Galactic Stock Exchange (multiple Merchant jobs and a whopping +20% TV bonus), the second tier admin cap booster and Adaptive Ecology (to allow terraforming planets). The latter in particular is not to be underestimated, since terraforming our planets will not just reduce pop upkeep, but also significantly affect amenity use, which in turn means a huge pop in happiness and stability for even more general job output. Don't forget to send a science ship to Assist Research on every planet with a lab. This will train replacements for your lead scientists when they die and provide a nice 10-20% science yield boost for the planet.
Ascension perk picks 3+4 are Master Builder and Galactic Wonders. If you see that you'll be able to get slot #5 early, you can also squeeze in Engineered Evolution. Probably a good time to go into Ascension Path choice here.
In the end you can pick whatever you like, but I am a big fan of Gene Ascension in Megacorp. Fertile grants strong pop growth in combination with reduced housing requirement, exactly what we need. We have a lot of science labs, so Erudite is very effective. Synth Ascension synergizes very poorly with Agrarian Idyll, since Farms are also the biggest housing providers for our empire. Psionic Ascension is perfectly fine, but I think the extra growth speed from Gene Ascension is worth more than access to the shroud. Even more so since we don't plane to wage any wars anyway.
...well, we might not plan any, but there is a good chance that the Khan will awaken at some point. To prepare for that and the upcoming Ring World construction, you'll want to increase alloy production to around 50-75 per month. Upgrade the alloy plants on your home world and the ones on your Habitats to get there. Also, by the time the Khan can awaken (year 100+), you should have the Citadel technology. If there are any Marauders in moderate distance to your empire, upgrade the Starbases towards them. A fully weaponized citadel can single-handedly defeat Khan fleets if your tech level is sufficient. Anything more than that (or attacks in quick succession) and you'll need to build an extra defence fleet to assist it. General rule of thumb: The closer the Marauders, the more likely you are gonna have a bad time. If push comes to shove and your border defence is on the verge of being overrun, it is better to bend the knee than risking a drawn-out war and territorial losses.
Your big mid game goal for year 100+ is to finish Mega Engineering and start the construction of a Ring World. These giant buildings were good in Utopia, but they are even better in Megacorp. They now have size 50 (instead of the old 24) and come with unlimited E and F deposits. Now remember that we are running Agrarian Idyll and get extra housing from resource districts? Yep, you can completely fill an entire Ring World with something like 15 Farms and 35 Generators and you'll still end up with enough housing for ~200 pops. It will probably take you another 100 years to completly fill a single Ring World, even with maximized growth modifiers and constant resettling of excess population from your other worlds. One good way to employ all of these pops is to use Commercial Districts. Get the basic buildings going, then fill the last 8 slots with Commerce Districts and upgrade as many as needed.
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Late Game: Unleash and Win
Once your Ring World is up and running, your economy is mostly unshackled from resource bounds. You can start to upgrade as many buildings on your other planets as you feel necessary. Build a few more Habitats and spam more Alloy plants. To prepare for the Crisis, you'll want at least 200-300 alloy income per month and around 800 fleet power in the field. That is a lot. And it will require an insane amount of Minerals. There are two ways to get them and the ideal choice depends on your geography.
Do you control a system with a Black Hole? Congratulations! You got the jackpot! Go and build a Matter Decompressor. It will provide you with 1000M every month, now you can just do whatever you want. You don't own a Black Hole? Well, looks like you'll have to go with a consolation Dyson Sphere and trade for Minerals instead. This will get you far less, so you might not want to upgrade everything on your regular planets.
Right, so now you have the Minerals, but what about that fleet limit? Well, go and build a Strategic Command Center. It is utterly broken right now - 200 fleet cap, 15 starbases, 12 (or even more?) extra defence platform slots on all Starbases and +15% sublight speed. If you turn all those extra Starbases into Anchorages, you should get another 400+ fleet cap from that.
Also, Genemod your species! I recommend the following as your final design: Erudite, Fertile, Industrious, Thrifty, Non-Adaptive. Don't forget to build Clone Vats on your planets and Habitats to get another 33% boost on pop growth.
When it comes to tech, you'll soon run out of regular stuff and end up with repeatables. Get economy ones when it seems necessary, otherwise focus on Admin Cap increase and Military stuff. I had games where I ended up with Admin Capacity 30 by year 220 - really helps to keep your tech costs in check, and it's not like Social has all that many good options anyway.
The last two Ascension perks are wild cards of your choice. If you have a ton of large planets, World Shaper can be quite good. If you have a lot of small planets with plenty of Deposits, consider Mastery of Nature. If you want to show any potential enemy who is boss, go with the Colossus. If you fear the crisis (and you should!) you can pick Defenders of the Galaxy. If you fear an Awakened Fallen Empire or the War in Heaven, you can go with Galactic Contender. I usually go with Defender of the Galaxy + Colossus Project.
Fleet wise, I aim for 2-3 regular combat fleets with 2 Titans (one for boosting/inhibiting aura, one with repair aura), 5 Battleships (Arc Emitters, Disruptors), 5 Cruisers (Neutron Launchers, Plasma, Stormfire Cannons), 5 Destroyers (Neutron Launchers, Flak, Point Defence) and 28 Corvettes (Stormfire Cannons, Torpedoes) - but these are by no means optimized. The remaining fleet cap is usually invested into pure-corvette stacks for rapid response. To help with that, you also want Gateways in every shipyard and border system.
So, what could go wrong here? Well, Awakened Fallen Empires are actually pretty darn strong. Each of your fleets should have around 100k fleet power, but they might actually be able to rival that after they awaken. If they do, it's best to play defence-in-depth. If they manage to breach our outmaneuver your defence lines, fall back and attack their fleets where you can outnumber them. Build Fortresses and Shield Generators on your planets to prevent them from passing deeper into your empire. If you can, strike directly at their home worlds - they have insanely powerful buildings, so once you can take out these planets their economy is sure to collapse. Ideally you want to take them for yourself (they can easily provide the same amount of resource as half your empire combined), but sometimes a colossus is the better option.
There is also the unfortunate case where the crisis might spawn inside your empire. If that happens, there isn't really much you can do. Again, try to prevent them from spreading by fortifying planets with forts and shield generators and attack them when you find a good opportunity. If you are lucky a nearby Fallen Empire will awaken and assist you. It is gonna be a war of attrition and it will be brutal and if their initial spawn is inside your Ring Worlds system, you might just die regardless. Otherwise, you should be able to single-handedly defeat the crisis on a medium sized map. You'll take quite a bit of damage, but with Defender of the Galaxy it is usually just a question of time until your economic might will crush them.
If the War in Heaven happens, you'll end up in a giant clustertruck. Create or join the league of neutral nations, then hope that your fellow AI friends are good enough meat shields to weaken and draw out the Fallen Empire navies. Your approval will be devastated by the war (since you technically declare an offensive war on them), so be prepared for some serious drop in your economy. The best option to deal with this whole mess? Take over the home worlds of the Fallen Empires, use the Colossus on anything else until it is over or you feel you have won and don't want to spend another 50 year cleaning up the galaxy. Your choice. There is a reason why I am playing as a Fanatic Pacifist. War is such a hassle.
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Conclusion: It's good to be a Fanatic Pacifist
I hope this give some insight on how to play. If you have any questions about particular details, feel free to ask below.
