Multiplay would be nice, but something that would be very, very needed, if it was over a network of some kind: a "talk" command. Should take very little time, and just allow you to send a message to the other marines. Seeing what others do and getting a special message upon something signifigant happening to another player would also be nice. IE:
Bob radioed: "Let's get on with the killing!"
Corpse's screams eminate from your radio as he is killed by a Hell Knight...
Zigmud suddenly grabs a chainsaw and screams "BLOOD!! BLOOD FOR THE BLOOD GOD!!!"
Ad infinitum.
Being able to share maps would be nice as well, but sight ranges shouldn't be seen by other players. Comp Maps would reveal all of the map for all players, of course. This, of course, assumes you're playing co-op.
Chat would be important for things like "Stash the shotguns - there's a metric ton of barrels ahead!", "I found another blue armor - anyone need it?" ect.