Tch, why conquer space for anyone else? Why march under the banner of the Krynn Syndicate when you could be The Confederacy of Ian? Who could stand up to the economic power of The Michael Empire? And imagine the terror enemies would feel lining up against the Aliciance fleet.
You can now make that happen in Galactic Civilizations III, as the latest update on its way through Early Access added custom factions, not to mention mod support.
It's a fairly hefty update, also bringing bigger map sizes, rounding out the select of base factions, adding more faction abilities, and boshing in a new planetary invasion system. You could crudely fling soldiers at planets to conquer them, but there are fancy options like converting folks with propaganda or simply wiping them all out with biological weapons. Why is everyone in space so mean? That's The Confederacy of Ian for you.
GalCiv 3 is slated to expected to leave beta and properly launch some time in May. Until then, it's on Steam Early Access. 4X games sound like an interesting one to play in Early Access, as the game may change a fair bit as new systems are added and tweaked. Though if you don't want a self-modding game, yeah, you don't have much longer to wait.