i actually dont see any issue with zombies just "spawning". lets face it, especially in MP, at some point the map is gonna be cleared of zombies. what long term motivation is there to keep on playing then ? a good thing here would be, from day one, a constant spawn of zombies that move across the map. like the number of zombies outside the mall in Dawn of the Dead ... they just keep on getting more and more...i dont need an explanation where they came from.