The overall concept is the game is assymetric singleplayer 4X where the player controls the Empire. Important part is that AI is not simulated the same way player is (they have their own goals and do not "play" the game and have no "victory conditions" and can not "win" the game, they just do their own stuff and can prevent the player from wilnning ba a "coincidence"). So, I don't want 8 empires where the player run 1 and AI runs 7. I want 1 Empire run by the player and 7 weird aliens or some other AI factions that do completely different things. A bit like "AI Wars" I suppose (but I have not played that one :D)

So, my initial idea was the player starts as an emperor and the galaxy is rather empty (the aliens will invide the galaxy from the "outside of the galaxy"). And there are terran pirates and rebels (so internal imperial struggle is a part of the game).

I wonder if that's the best model... Maybe incude aliens inside the galaxy and make them weird enough? So the player, kind of, conquers them /defends from them?

if you can make different (and interesting) the various alien's goals it can be very fun, a game in the game to "see" what alien races want to do

At the moment I have 2 aliens from outside the galaxy:

- methane breathers - they don't have the homeworld (it's in another galaxy) and they spawn at one edge of the galaxy and try to colonize slowly, they try to avoid conflict but they terraform planets to methane atmosphere making these unusable

- trans dimensional aliens - end game threat, they come in and don't conquer but annihilate all life, need to be stopped at all cost

And 2 native to the galaxy:

- parasites - they have a homeworld somewhere at the outskirs of the galaxy. It houses a rase of toxic, hostile, uninteligent race. Once every 100 turns they go on a rampage and infestate the nearby planets one by one (then they go back to their home planet and hibernate till the next cycle), their home planet is extremelly hard to conquer.

- the elders - a race in decline (negative breeding rate, they most likely will extinct somewhere in the mid game), they used to rule the galaxy, now they are dying out and are passive and not interested in anything. They are quite powerful. When they extinct (thae player can speed it up of course the player can scavenge their technologies and assets.

But I don't know... Maybe add more aliens that are native to the galaxy and need to be get rid of?

what about some kind of spores that travel through asteroids and randomly crash on planets and start to contaminate?

Well, let's ignore alien types for now. I ask primarily for the basic mechanic.

I have 2 options:

a) The player is the Empire, the galaxy is empty (no other empires/races, with small exceptions for flavour). The threat comes via the edges of the galaxy, like 4 alien races that try to colonize it from the borders. But they do not have homeworlds in the galaxy, the player can not eradicate them, only stop (maybe there will be an option to build some trans galactic warpgate/superwerapon so the player can get rid of them, but no actual conquest of their homeworlds is possible (since these do not technically exists)).

b) The player is the Empire and starts in the middle and there are several weird aliens in the galaxy (with homewhorlds) and they colonize nearby planets (with a limit, they don't want to grab it all) and then send waves of fleets vs the player (based on how annoyed they are).

I need to choose one...

I try to figure out the map (in sense of aliens/threats) mechanic.

Maybe a "squeezed bubble"? Like you (the Empire) start in the middle and expand outward, the major aliens start at the very borders of the map (they enter from another galaxy) and they expand in your direction. And the struggle is how big your central "bubble" is (you get bonuses near the center - proximity to the imperial capital; major aliens get bonuses near the edges - proximity to their homeworld galaxies/portals/warpgates).

There would be also minor aliens that live scattered inside the galaxy (and pose medium threat only).

The victory condition (one of them) is to get & hold a certain number of planets (not all of them).