Well the thing about zombie games on this site is that there's just so many on this site that pretty much everything to do with zombie survival has already been done. The tricky part is coming up with something original. Here's a couple of ideas I haven't seen done yet.
1. Intelligent zombies. I've seen faster zombies, stronger zombies, weird, alien like zombies and even zombie dogs, but I've yet to see an intelligent zombie on the site. Zombies that don't just wonder around aimlessly munching on brains, but actually plan how to catch their victims and where to look for them.
2. Talking zombies. I know it sounds stupid, but this is more or less the same as my last point where, basically, all the zombies do is wonder around looking for brains. It would be cool if they actually evolved into some intelligent form of being that could talk and communicate with each other like a normal person.
3. Transforming zombie. Usually in a good zombie story game, the main character has a friend or relative that turns into a zombie, but usually that transformation happens very quickly. One second they're a person but the next they're a zombie. I think it would be cool if there was a transformation sequence that took a bit longer, where the person was half way between being a person and a zombie. That way killing them would be more difficult for the main character.
4. Zombie children. Has been done to some extent, but how freaky would it be if the main character got stuck in an orphanage or something and all of the zombies were little kids? It's like the little girls from the Ring or The Exorcist. They're kids, but they're scary as hell!
5. Humor. The main guy is surrounded by zombies and the only way to survive is to distract them by dancing the Thriller... Zombies love Thriller! ^_^