"Competitive" Zombies
A gamemode (teams of 4 or 5) that try to survive the longer than the other team against waves of zombies. I do think the zombie waves will need to be buffed though, or else the match will last a very long time. Also, you can make it so a dead player can act as a zombie, or increase the number of rounds it takes to get better weapons (this will make the match length much shorter)