A Warhammer 40k Civilization type game with planet and space layers. All the major factions will be present, the Imperial factions divided into 4 major parts:
The 'Regular' Imperium of Man is represented by the Guard and plays like any normal civ but with more heretic burning. Kinda weak in the beginning of colonization but economic powerhouses at the endgame able to produce enough to drown entire cities with bodies and tanks. Pretty decent all rounders, just need the numbers.
Space Marine Fiefdoms are small but produce way more, think something like militarized versions of Venice. Extremely powerful individual units but you won't have a lot of them and support from the regular Imperium helps a ton. Much, much better all rounders.
The Church is represented by the Battle Sisters. Their forces are divided between the elite sisters and their lower fiefdom PDF rabble. A lot like Bretonnia from regular Warhammer. Plays alot like the regular Imperium but their cities are cleaner looking.
The Mechanicus is really more of an addon, as they only get 1 city like any trader civ, but that one city is enough to power their entire faction. Very few early units; in fact, only one: the Skitarii. Servators get unlocked to handle specific combat tasks later but for the majority of the game, you only get those red suited chrome boys. Then the endgame happens. Deploy all titans on a city and laugh manically through your drive thru window speaker you call a mouth.
The Tau would be the equivalent of any of the religious civs. Infect Imperium cities with the Greater Good and freedom and watch them happily convert to your side. At least until they get self nuked. Excellent ranged combat, best in the game in most unit categories, but melee ranged units murder them. Even Guardsmen that get close enough to use the bayonet move.
Orks are well, Orks. They have the ability to migrate their 'cities' closer to enemy territories in order to get closer to the fightin'. Quickly amassed, tough units. Very few early game tricks up their sleeves. Lots of cannon fodder to soak up sieges while you pound on cities.
The Eldar; in any other game with covert options, it'd be these guys. That's not to say they don't have good units, but each one is very specific to a task. While the other factions can swap heavy weapons attachments to units on the fly, with the Eldar you have to create a whole new unit to do so. Also, they can relay units from any city to any other city really quickly.
The Necrons. I really don't have any ideas for these guys, other than notoriously tough units that take forever to build.
The Tyranids, these guys are also hard to come up with civ options for. The ability to infest cities, sure, and quickly built cannon fodder that needs to be within X tiles of a leader unit to gain full combat strength.
And finally the forces of Chaos, the 'regular' Cultists and the Dark Eldar. The regular cults are represented by mass expendable cultists early on, and graduate into Chaos Space Marines mid game with Daemon support later on. The ability to weaken city defenders with cults and open up temporary warp portals to quickly get units to the front.
And the Dark Eldar. The ability to raid cities for slaves to boost output. The ability to use warp gates to quickly get units around. Same Eldar flavor with some spice thrown in.