A real zombie outbreak simulator would be great.
I actually started work on an ArmA 2 mission (for now suspended due to heavy CPU usage resulting in a crazy low fps) that fills an entire island with zombies, civilians and which i dressed up with all kinds of usefull stuff to find.
People would spawn in one of four different locations on the map, have to find their own way without a map gathering stuff along the way and eventually escape from the island.
Preserving ammo is key because the zombies are everywhere. Interesting locations would be military bases (filled with zombies but lots of ammo and better weapons, probably a vehicle or two etc), radio towers (allowing you to call in a rescue helicopter, though most radiotowers lack the signal required to do so), airfields, harbors and also hospitals in cities where you might get a vaccine incase you might get bitten.
Other than that there will be various human outposts like church refuges, a research centre that captured zombies to study them in an animal pen (day of the Dead anyone?
), military outposts equipped to exterminate the zombie threat and burn the bodies, and stuff like random carcrashes in the middle of nowhere.
Also i wanted to hide portable radios throughout the map that will play static messages when you come near them before signal is lost (deranged priest uttering stuff like "when there is no more room in hell, the dead will walk the earth", to reporters giving an update on the latest news with some hints in there like casually mentioning a hospital in a certain city or a refuge centre located somewhere).
However i put it on hold untill i get a pc that can actually run it