I just answered in another thread that I am very partial to the zombie apocalypse, and extract an inordinate amount of fun from games related to it, so my answer will be biased: there are not enough Zombie games out there.
I agree with Fat Man Spoon and his post [http://www.escapistmagazine.com/forums/jump/9.135769.3058245], I recommend experiencing other games that contain zombies, so you can get a better idea of the possibilities, and gauge the need to send them back to the pits of Hell by removing them from a game with a ban or with a shotgun.