ideas....
1- spawning not by time, but by how many "zombies" you tag. (this will be important because you don't want to start level 50 with 500 zombies all at once, it will lag the server, it needs to make it so that maybe 10 spawn after killing 9)
2- Health and Health packs/regen. I know this was talked about for single player, but i feel like if the zombies tag you (not sure how) you should be tagged out for good: 1 hit you're out)
3- Multiple zombies with different AI, like a melee type zombie that moves faster, a pgp type zombie who shoots slower, a Giant zombie that you may have to shoot 50 times before it goes down.
4- a system for unlocking better weaponry? or just go with regular "find a gun" method.
5- a way to unlock rooms? not just by button but by purchasing perhaps? much like in COD or mayhaps by kill streak?