Games that could be turned into a good RTS game.
TF2 (Team fortress 2): Why?, it has units already, it has hp already, and you already have bases... maybe we could turn sanviches into resources.
L4D (Left 4 Dead) Why? Yeah I know this is another Valve game but having one player make swarms of zombies and another build a ton of forts would be fun.
Gears of War Why? Interesting locations for an RTS, would like to see what would be done with the cover mechanic.
Brutal Legend... see what I did there?
Castle Crashers: The cartoonish characters and world would be a great setting for an RTS.