Ok, I know anything multiplayer is exceedingly unlikely to happen, but this sort of thing would make me giddy, so I thought I'd throw it out here anyways. It would probably make a good DLC.
There could be a coop game mode kindof like DoW2's Last Stand - each player has their soldier (or perhaps 2 or 3). Before the game they're outfitted/stats are selected, and the players are bit against wave after wave of increasing strength. Continuity/progression on your characters would be nice, but I'm not sure how that would be balanced so that newer players aren't uselessly inferior.
Or perhaps it could work such that the coop takes place on community created maps, which can have set objectives, or just the above wave spawning.