Giant Oversized Mutated Zombies that you can climb around on and attack. Climbable places or in fact Climbable world in general there has to be something to climb cause jumping over things is too mainstream.

zombies that mutate the longer they survive. Causing new types of zombies to spawn.
I'd personally like to see a fallout type experience when it comes to zombie games.
Where you have to scavenge to survive.
I also would like to see a zombie game where the goal isn't to kill everything that moves, but rather you have options for escape.