Sins of a Solar Empire + EVE Online = A crazy rts where most of the units are actually your buddies zipping about space fighting your battles for you.
Halo Wars + Halo Reach = You're the general deciding which AI controlled units pop into play, your friends play as spartans and experience the game as an RTS. Like Savage, but it doesn't end up sucking to be the general.
Marvel vs. Capcom 3 + Mortal Kombat vs. DC = Come on, Batman fights Ryu, Scorpion against Spiderman. And while we're talking about it...
^ + GTA IV (Or any other sandbox game with a huge city and some destructible environments)= Huge arena style fighter where you have Iron Man throwing Superman through a building, then bringing said building down on top of Superman.