Okay, this is going to seem cliched but here's my pitch.
GTA with zombies.
An open world sandbox, third person shooter set within a San Andreas like map (several cities with outlying countryside). I can see it now, you're driving out of the city as night begins to fall. You look at the gas meter, shit, almost on empty. You scan the open highway and see a tiny gas station in the distance. You pull up to the pumps and get out. Everything is quiet; too quiet. You're car is filling when you hear something in the distance. Shit, shit. They are coming, the car is filling. Do you jump in and drive off and risk running out of gas in the middle on no where. They're coming, closer, closer.
Anyway, I think that type of open world, survival horror type game could be really cool. The way I see it is that there would be a story mode, where ammo and resources are VERY hard to come by. Seriously, you'd be completing mission for ammo. There would be typical missions such as, oh I'll give you this gun if you rescue my daughter from the warehouse, etc etc. And you'd work to build your rep with survivor communities by completing missions and donating supplies, etc.
Then there would be a free play mode where you can enable tons of zombies, all upgraded guns, etc etc, where you could just blast away zombies and come up with creative ways to kill them and just have chaotic fun.
Anyway, that's my take.