There has been an outbreak of a certain bioweapon that keeps brainfunction intact after death. I.E.: zombies.
All cities have walls to keep zombies out. Traveling goes with armored busses with gunturrets and imune elites to guard them, always in teams of 2. The imunes are those who were infected, but did not die.
You are one of the imune guards. You are with only a handfull. You have no choice: you are the only hope at a normal life. You start of traveling as a guard. When you come to the next city, things go down the crapper and there's a breach in the wall. Apperently the zombies are gradualy getting smarter. The found ways to use weapons against you. Busses get blown up, all people killed and absorbed in the ranks of the dead. You protect the city, wall is repaired temporarily, things appear safe. Then a bus sends an alarm.
You are sent to help a fallen bus. You go out in a armored jeep, arive at the site, and see horifying things. People are being torn to shreds, eaten and raped.
YES, RAPED!
The smarter zombies are generation 2 zombies. They need live women to reproduce. The men are killed of on instant. Since there's nothing left to save, you kill of the newly infected, then return to base.
At the base a signal has been picked up: a town has been invaded. You and some more teams set out to the city, arrive too late and find only zombies. In the townhall you find some papers on zombie behaviour. One of the papers describes the evolution of newer generations of zombies.
There are people who know more. You and your group set out to the lab where the papers came from to learn more. That's when stuff get's rolling. There are some scientists alive. They declare that the head of the zombiedepartment has gone mental and has been making the "perfect zombie". He is the one that impregnated the first woman with zombie-altered DNA.
This guy left with an armored bus and a pod with something in it. He busted through the doors and let all the zombies in.
You travel from town to town in a sandbox-fashion. You gain new weapons, bio-upgrades(NO BULLET-TIME!) and improve your vehicle. The story folows the path of the scientist. Why, how and where. The sandboxing consists of favors, going to facilities/houses outside the walls retrieving stuff and finding out wtf is going on.
Idea is that your teammate can be controled army-of-two-style. The vehicle can be driven in any position. Multiplayer is an option.
Might be a bit stupid, but in my head it looks awesome. Destroyed cities, forrests, no sewers, military bases, awesome vehicles, great weapons, multiple kinds of zombies, mixed breeds of zombie-animals...
Aww, the ideas, the ideas...