dunno if this has been mentioned before but... the original perfect dark on the nintendo 64 had a counter-operative mode for the normal single player mode: In which one player plays the missions as the main character while the other takes over the role of an enemy, including their weaponry and lower health and attempts to stop the other player. Imagine this is in the context of the Halo Reach campaign, you could have 5 players as the spartans pitted against another 5 taking the role of the covenant in any particular mission. Kinda like the versus mode in Left 4 dead. thoughts??