I've thought of the best game mode. 16 or so people, and you have to survive for 30 minutes or an hour or something and then a helicopter arrives.

However, one player, or maybe up to four players, I don't know, are secretly designated Psychopaths and their objective is to be the only survivor on the helicopter. And keep the big red "PSYCHOTICWALRUS HAS DIED" text from the last game for that mode. Man that would add atmosphere.