A coop gametype would be awesome. The single player is so great in this game.
Imagine mission based coop games, where traps would imprison a team-mate and you and others have to go save him. Or imagine a sort of anti-coop where each chooses a side - marine or hell and helps minions / soldiers on either side complete a mission/task. Multiple levels of coop would be great too. Complete so many tasks till the next level, then it loads a new map. The PDA could be used for maps and markers, and even able to communicate with team-mates and set waypoints. The keosks could provide new missions or other things (as suggested by my friend Rroff).
Lots of possibilities for coop. Even a mission randomizer that has a bunch of missions and maps and can mix them based on a cookie cutter format to provide tons of variations and possiblities.
Just brainstorming … 