IDK if its possible but would be cool if it started with 1 zombie, then when he managed to kill someone they would respawn as a zombie too.
Rewards would be given to the people who stays alive the longest.. (and if you want any reward, even as an early zombie, you had to stay to round end)
Zombies could have increased stats in the beginning, then diminishing over time as more people become zombies and then respawn.
So basically it would be more and more zombies, and less and less normal players. The next round the last player to die would be the first zombie.
(The very first zombie could be some kind of zombie "king" with a new model and special abilities.