Left 4 Dead with Dawn of War. A coo-op, objective based survival horror game, playing as space marines, fighting your way through a chaos-infected city. Of course, you need more enemy types. Some with lasguns or something. The powerarmor would also explain why you characters could take such an ridiculous amount of damage, and the bolters would fell most enemies with one shot anyway. As for special enemies, the variety of the 40k universe gives us plenty to choose from. Chaos Terminators, Demons or Necrons.... well, i think it would be fun.