What about a prison?
They always have prisons in FPS games then fill them with monsters or giant ants or whatever - you never see a prison full of zombies, which is a bit weird if you ask me, think of the possibilities:
Cafeterias, gyms, shower rooms full of zombies.
Guards would be zombified too so would spread the infection all over the place.
Survivors trapped in cells, keep the zombies at bay while trying to rescue them.
Trap zombies in cells, and other lockable areas.
Prison yard with zombies wondering about.
Lookout towers to snipe wondering zombies.
Lots of gun and ammo stashes.
Maybe you could have a guards keys and be able to lock and unlock most doors, would make it quite strategic.
Escape the prison somehow, maybe have a couple of options or even a main character to rescue (prison governor or something).
Get me John Romero's number immediately