I'd make a game much like s.t.a.l.k.e.r. and the more recent fallout 3. A 1st person shooter with very heavy RPG influences. But mine would be primarily involved with zombies. (mostly shamblers) With humanity surviving in small fortified communities kinda like in fallout 3. There'd also be a large hi tech city run by a fascist government that houses the higher social classes, treating anyone outside as animals.
The zombie infection would only occur when you actually die. So the player can get injured by zombies and survive. Zombies are rotten though, and infected wounds might be a constant problem like radiation poisonong is in fallout 3.
Like stalker, you be a generic scavenger. An adventurer type person who goes out into the zombie infested country to collect stuff like food, weapons and valuables. Apart from undead hordes, (wich would be deadly enough youd have to take special care to avoid being surrounded.) there would also be raiders, people completely devoted to pillaging other survivors as well as surviving zombie attacks. And soldiers from the hitech city who attack scavengers on sight while fighting the undead.
Although there would be a main storyline thorughout the game. Id want to games focus to be in day to day survival. So that any gamer who turns on his computer and starts playing, will eagerly look forward to 'how far he can make it inside that city/forest/underground today'. With factors like your fatigue and hunger to keep in mind. (Why have a 'sleep' option if you never need rest? Why have food in the game if you never have to eat?) Is your character getting tired? Then he wont run as fast and long, and your ability to fight will be limited. Better make sure you sleep in a safe place, or you might wake up with a zombie hunching over you. Upon death, you return to your 'hub', but youll have lost everything you collected on your last hunt.
There'd be multiple maps. Each one as large as the capital wasteland (hey, a man can dream) but every one being significantly different from the other. It could differ from a city setting, a wilderness like mountains or a forest, and the country side. All maps can be revisited with a special vehicle (a train, maybe)
speaking of vehicles. Your character can access and use a variety of vehicles he comes across. Cars, motorcycles, buggys etc. Kinda like GTA, they have a limited lifespan so running over too many zombies will cause your vehicle to break down eventually. And they run out of gas, wich is a precious resource so youd think twice about using it carelessly.
Thered also be character customization. Apart from a very detailed appearance generator. Your character can specialize in different ways like a gun expert, close range powerhouse or a stealthy type focussing on remaining unseen and escaping dangerous situations. Though I wouldnt play one myself, id also work on a diplomat type who makes profit through trading and can get alot done through conversation.
Maybe around the endgame (you character might be wearing a hi tech suit of armor and have badass weapons) you'd be able to take on zombies without a care. The enemies scale up as your level does though, wich could involve existing zombies getting faster, or the appearance of new types of undead that are more dangerous. (ragers, spewers, etc. maybe resident evilish mutant monsters)