Mid and end game relevance

Thing is, there are lots of issues that can lead to mid- and late-game stagnation in the 4x genre.

Micromanagement hell is definitely one of them (and one of the only real flaws of MOO2, which this game will need to properly address). Possible solutions are Stellaris-style AI control of outlying sectors (but I don't think that would really work), or limited build slots which make management a choice, rather than an endless queue. A good solution to this would be the holy grail!

The galaxy going from unknown > known also slows things down. The dev diary about the map made me hopeful that exploration will continue to be a thing throughout the mid game, though. To draw on Stellaris again, their "End Game Crises" did try to shake things up, so that's another option. Internal politics and revolutions might also make things feel less static - for example, the "Revolution" mod for Civ 4 was quite good at making the the game feel more fluid, even towards the end - new up-and-coming civs would dynamically appear, whilst large powers could over-reach and fracture into smaller states.

The final problem that leads to 4x gameplay stagnating is the snowball effect - with every victory you get stronger and stronger, so the victories get easier and easier. It's difficult to counter that, without it feeling frustrating for the player. If there is a good model for civil wars and dynamic politics, like the Revolution mod, the snowball gets countered a little bit. I also wonder if there should be some kind of "civic threat" mechanic to represent the fact that small empires will have more internal unity and fight harder (to the point where, if they are defending their homeworld they will fight like demons). It should be an extra challenge to conquer a homeworld completely. Whereas bigger empires will naturally feel less of an existential threat, and will start to get complacent, like medieval China or Ancient Rome... and so more likely to indulge in civil wars or other internal power struggles. Maybe smaller "threatened" empires will be more opportunity to research experimental superweapons or have more radical leaders come to power - basically allowing interesting "catch ups" for players that fall behind, and new dangers for powers that get too big.

 

Those are some excellent points you bring up, the question, as always, is how to address these points and still keep the game fun.
Moo2 never became micromanagement hell for me, granted as the game progressed there were more factors I needed to consider but thanks to the mechanics involved I could easily put most of my colonies on research and set the build cue to trade goods. This would then allow me to focus construction on only my richest planets.
The run-away effect however, when some empires totally dwarfed the rest in economics, research and production always left a bad taste in my mouth. Perhaps one way to address this is by introducing a non-intrusive mechanic similar to a "National Guard"
If for example, the game allocated some ships to a "home fleet" that remained in-active and not subject to player control until certain parameters were met (home system attacked, loss of the main fleet, etc.) then we could get a better defensive effort from one of these lower end empires. I do not believe that this by itself would alter the course of the game but rather that it would extend it. Wars would now need a few more battles to be concluded and that is not a bad thing.
Similarly some events could be used as well that would be triggered by falling too far behind. (Legendary commander arrives with a fleet that was thought lost and brings some tech as well).
Once again, the goal would not be to alter the course of the game in an arbitrary fashion but rather to level the playing field some.

 

EU IV tries to stop run-away expanding empires using the coalition mechanic. If you expand too much, neighboring nations will form a coalition against you. If you attack one, they all join in. At some point it stops mattering and you can stomp all over them anyway, but it's a good mechanic that forces you to be careful. Take too much in one war early and a coalition forms that certainly makes it harder to expand further.

 

An interesting proposal. I imagine there would be some differences with the concept however.
In EUIV you do not have to factor in racial characteristics in the likely hood of a coalition forming.
In a space based 4X it would be almost impossible to know in advance a players neighbors and the characteristics of those neighbors, this mechanic may come off as forced.
Imagine two neighbors, clearly hostile to each other (possibly repulsive), forming a coalition without clearly being threatened.
This was the sort of thing I faced when playing Risk when I was a young teen.
On the one hand it makes total sense to us humans.
Would it make as much sense to aliens that are very different from one another?

 

I agree that it would have to be modified by races opinions of each other, but even if they hate each other they still might work together against a common threat. Especially if they were threatened enough. It seems a lot of games these days are including "threatened" as a relationship along with the traditional friendly, hostile, and neutral. If I wipe out a mutual neighbor in a dozen turns and / or without losing many ships, anyone surviving who isn't my ally is going to be pretty threatened.

Per the other thread on Star Control mechanics, the Ur'Quan arrived and a whole bunch of other races basically formed a coalition against them after they enslaved 6 or 7 species. And the Vox, who were the definition of pissed off at the humans, joined the other side out of spite. So I think it could apply if done correctly. It would require tuning, but it's definitely a viable way of slowing down steamroller expansion.