I would like an strategy game with real different mechanics for real different factions. Instead of just reskined factions with slight different mechanics, so playing with different factions migth be basically playing different games.
Universe at War: Earth Assault was kinda this. Shame it got shackled to the dumpster fire of Games for Windows which meant a swift death for the multiplayer scene.
Basic gist was, Earth gets invaded, super hard, and then you have 3 alien factions fighting over it.
First two factions were most similar to a classic RTS, the Masari were these sort of ancient aliens who built a base and trained units and that was mostly it, except you had a dark/light mode which would do wild shit like give your units massive DoT damage and enable certain units to fly while the other mode would give everything a damage shield and slowing attacks. There was a bunch of other stuff that got influenced by the mode you were in but I can't recall all of it now.
The Novus were a sort of robotic race, again, mostly classic RTS gameplay, except they would build massive networks of power poles across the map, and then use these to have their units zoom around trough them, making for incredibly good hit&run and raiding tactics. On top of that, since they were robots, they had this patch system which was a number of different faction wide buffs which they could apply to adapt to almost any situation, with the kicker being everything would shut down for a few seconds while it got downloaded.
The Hierarchy (I think that was the name) were the big bads, and they were the most unique of the three in that they didn't really go for base building. Instead they'd just call down giant ass walkers from orbit which you could then customize with a ton of hardpoints, ranging from weapons to unit production facilities, so you had this incentive to move around the map and use them to attack and harass. Of course that ran the risk of losing one of the stupidly expensive giant ass walkers.
Additionally both Novus and Hierarchy would scavenge the map for resources (your gatherers would go around sucking up cars, houses, cows, etc.) so you had to go out and control territory if you wanted to stay competitive.
Was a really cool game, made by Petroglyph before they kinda went to shit and started spamming out unfinished C&C clones of various flavors.