I always feel with Stellaris that it just needs
something and it'll be great. What they did with the populations was great, but it mostly fixed problems. The faction system is amazing but when things are going well its mostly that you set things up and then they just work. Utopia is also great in that it extends the feeling of discovery and growth into the lategame. But again, its a great system but you only need to interact with it when something big happens. Unity is also going to need its own rework.
There are two high-level problems I see with Stellaris.
The first is an identity problem between a 4X and a GSG. They've got the 4X systems down and the Paradox GSG systems down and that's both great. The problem is that those two have different end goals. Paradox GSG's all feature a single common thread of evolution and proxy/distant war. CK has centralizing/crusading, EU has enlightment/colonizing, Vicky 2 has industrializing/colonizing and world crisis, HoI has militarizing/the war. The point is you can spend all of an EU or Vicky game as Spain competing with the UK, and never once actually take any part of the British isles. 4X, on the other hand, have the common thread the map decaying from many small powers to single victorious power. The GSGs have that too, but people generally don't like it and the main reason they work is because the time limit means you've never actually conquered the world. Simply put, right now Stellaris just feels like a 4X with the brakes on. Whoever has the best colonization/research/production will conquer the map. But the Paradox GSG systems will make it really. Painfully. Slowwwwwwwwwww. In Galactic Civs or Endless Space if your time comes and the giant fleet is going to kill you, you just die because the war won't end and planets flip immediately. Once the balance has tipped the snowballing is faster. The developers need to make a decision: is this a forever game or a blobbing game? And they need to take a leap.
I kind of hope they pick forever game. The society roleplaying stuff is why I play this game and it doesn't really work as a multiplayer tuned 4X. If they do what it to be a forever game, they need to do a few things. First of all, they need more independent powers. A bunch of blobs that cut each other off? It just doesn't work. The only solution to that is to take land to free up paths, but then what happens is you become the blob that cuts people off. It should be more like Vicky where there's small powers that you can influence but if your influence wanes maybe someone else could influence them in the future. Or... something. It should be like the real world where there's a lot of countries in general and small countries become economic and political satellites to the bigger powers.
Second of all, we need trade dammit. That would greatly discourage closed borders. It would encourage constant low level interactions with your neighbors. And it would add some variation over time to both diplomacy and economics.
Thirdly, we need a way to evolve not just from a weaker thing to a stronger thing, but a thing to another thing. Think EU4 where yes you get more technologically advanced but you also have the growing pains of the reformation and the shifts to gameplay that come from you and your rivals all getting national focuses. Or Vicky 2 where yes you become richer but to have more advanced factories and militaries you need more and different materials. For example the shift from needing oil to not needing oil. A lategame Vicky 2 nation is a fundamentally different beast with different priorities than a midgame one. An lategame Stellaris nation is a midgame Stellaris nation with extra bits and a much higher fleet power. We need... something. A complicated economic system, social changes that cannot be avoided and must be reacted too, civil/technology changes that place great stress on the nation (by requiring extra resources or changing which other nations like). Something. A metamorphosis should begin from the time the borders meet to hour 50+ of the playthrough. I don't know what it should be but we need something. This being Stellaris there should be more the one option.
I would actually like it if there were at least two options. To be clear I'm not entirely sure by which systems the ideas I'm about to present should be accomplished. But, you should be able to do the usual Paradox metamorphosis of thing to stronger-but-more-awkward/specialized thing. But you should also have the option of ascending towards being something other than a nation. For example becoming an enclave or maybe even achieving singularity and becoming an ultrapowerful OPM. You could be ultra-powerful for your (tiny) size, and then have prioritizes involving the larger galaxy (enforcing your ethics, making deals, controlling specific resources or something) and that would be how you become more powerful. Maybe even have an off-map growth method like CK2's republic estates. Both of which would be totally optional of course. But I mean, if you want to ascend into all living as brains in jars in a perfect simulation and abandon/release all your colonies, doesn't that just mean more fun for everyone else? I think you should be able to evolve into something ultra-tall or ultra-wide. Being ultra tall would encourage you to merely influence other nations rather than directly control them (hopefully with some way to get strategic resources under their control by way of ordinary trade which gets easier if they're under your influence). For example you could become a super-feudalist and only be able to control your core sectors directly but have great benefits for vassals. Or you could become a super-powered bureaucratic technocracy built around a giant complex in your homeworld, greatly expanding your research (and maybe even ascendence perk) options but increasing the ethics divergence for distance from capital and reduce economic efficiency. Becoming ultrawide would be essentially become the victory push; you become the all-conquering purifier/devourer/conquerer, and the other nations begin more and more reacting to you as a lategame crisis. Either you win and everything is you, or you get pushed back and probably lose.
What I mean by this isn't necessarily an explicit evolution. More of a path that, by its sum parts, produces an outcome that has both bad and good things. Like how colonizing in the other Paradox games both adds a new axis to gameplay but also comes with costs (opportunity costs, competition for colonies, expense of maintaining them). Other nations would of course have all of these options. So part of what would make the lategame interesting is seeing which way they would jump.
The second problem is the feeling of discovery. A lot of people have commented that they had fun for the first X hours and then it gets stale. That's because In the beginning you're hunting for worlds, figuring out what where the other nations are and what they stand for, wondering how powerful they will be and what their relationships will with you. Wondering in particular what major plotlines you'll encounter (long events, particularly interesting worlds, space monsters). But once all that's tied down, you... pretty much know everything. And that's sad, and its boring.
Right now Paradox is approaching the problem by adding extra scripted content and extra ways to build tall. And that's absolutely the right thing to do, but its not enough. Building tall is too much within our own control, and scripted content is just. The games are so long, scripted events can only be so much of our time. Normal gameplay systems are interesting because of how they interact with each other. The Horizon Signal chain is amazing but its not interwoven with the other mechanics and its scripted to a greater degree.
What we also need is a concept that the larger galaxy could go places we can't control completely. For this, I would propose 5 ideas.
1. Megaevents/crisis that aren't do-or-die. Some examples: a new type of species has emerged, they live in gas giants and have completely different mechanics. A wave of spiritualism sweeps the galaxy. A brilliant inventor discovers a powerful new technology (which is normally banned outright) and reveals it to the galaxy, allowing everyone to research it, but the only way to actually use it is with a strategic resource that is also revealed at the same time. Giant red stars across the galaxy are going supernova and may destroy their entire systems and cleans any neighboring ones if the galaxy doesn't cooperate to complete a task. A strange race of migratory aliens comes from outside the galaxy, peacefully establishing multiple new nations by terraforming worlds the galaxy thought uninhabitable. That kind of thing.
2. There could be additional axis of morality that emerge as the result of technological development and exploration. For example something like immortality versus death emerging as a result of longevity tech. It would go something like "utopia is everything staying exactly the same < everyone should be immortal < people should increase their lifespans > people should be restricted to a natural lifespan > people should have their lifespans artificially shortened. Everyone starts neutral because you don't have to seriously think about an issue like that until its the future and you actually have to make the choice. You could also have something like wholeness versus separation (as in, the extent to which technology is allowed blur the line between where one person ends and the other begins, with hive minds being the extreme), time spent living in space versus living on a planet, I dunno. Just like, the ethics system is good for capturing morality in normal politics. But what about blue and orange morality in space politics? It could add some motion to the faction system.
3. Moral/practical issues which emerge that are semi-sideways to ethics, and that everyone makes a choice on. An example would be something like "should megafauna be protected or not." Spiritualists/xenophiles/pacifists/egalitarians might err on the side of protection, but that's not a core issue for any of them so you could conceivably make whatever decision you want. But the point is, you don't make the decision until you encounter a megafauna. It could emerge as an issue early on, you make a choice, then later on everyone's relations are effected by which choice they made. If a lot of nations decided to protect or exploit/hunt then that could cause conflict and perhaps even one side could persuade/force the other to accept their view and in that way consensus is reached. Thus one player could have a galaxy teaming with megafauna and the other could have one where they're all hunted early on and the few sanctuary civs get subject to essentially poaching wars. Then the issue could be complicated further if a new strain of megafauna enters the galaxy. Other possible issues could be things like dangerous tech adoption, UN style stuff (can civilian traders pass through closed borders? Should fleet size be limited? Are robots people?), or maybe even galaxywide projects analogous to modern environmentalism (example: many stars throughout the galaxy must be modified with a special process or a few stars will go supernova and wipe clean their systems). This is a huge suggestion but do you see how this could result in interesting gameplay and an unknown factor?
4. There should be long event chains that emerge from inside your empire over the course of gameplay. Just something as simple as "a smuggling ring has emerged on this planet and we fear it could spread" or "our ruler has insulted the ____ faction greatly, what should we do?" And then maybe a couple larger events, but nothing too RNG or game destroying.
5. The last two paragraphs of the first problem where I talk about changing instead of just growing.