[Tyhm]

1) I still say Horror/Survival Theme is best; then you can have zombies, dinosaurs, aliens, werewolves, whatever.
2) The player doesn't become a zombie; the player is a ghost that possesses a new soldier that looks just like the last one (bring in another clone). The dead Character becomes a zombie, this way the zombies are as varied as the humans are. If the human had body.png, the zombie has bodyzombie.png; if the human had body7.png, the zombie has body7zombie.png...
4) Although it would be cool if there was a Crown of Orcus somewhere that let a player take psychic control of the hordes of undead...maybe they'd have a RTS style map-screen-level. But I don't know how practical that is, so maybe they'd just be able to shout commands at nearby zombies. "Attack towards this location" instead of "Attack randomly".