Level 3
Just a suggestion which any of you are able to slam, but would any of you enjoy a zombie mode?
Of course there would be massive limitations in a zombie match, for instance the humans would not have a primary weapon only the pistol with the bullets in their clips maybe have an ammo crate. The map of course would also have to be unqiue so that bases are removed and transportation etc. Zombies only have a melee attacks (R3), doors can be damaged using that attack. etc etc
Just had this though whilst playing with 255 other people and how cool would it be to try and survive with the constant bombardment of zombies. =D
Level 2
You talking about NPC zombies or what?
How about one platoon of players with guns and two or 3 platoons of zombies controlled by other players, and zombie classes perhaps? Some faster, some bigger and stronger, others with more health.
4 rounds and a full rotation so everyone gets to play as the humans and the zombies. Or eh, a completely original game mode where the humans all have 5 lives before turning into zombies and the two factions have to battle it out. Once a player runs out of lives they get turned into a fast moving zombie and get triples points for each kill they get regardless of what team it is.
Yeah... Never going to happen 
Although "Massive Action Game" doesn't mean that there can't be zombies...
