Try Zombie Fortress.
Blu are the zombies. They can only be Spies, Heavies or Scouts and can only use melee.
Red are survivors. They can be Demomen, Snipers, Pyros, Medics, Engineers, and Soldiers.
When a Red person is killed, they are swapped to Blu.
If the Timelimit is reached and Red has one person alive, Red wins. If Blu eliminates all Red, Blu wins.
Of course, the classes have be nerfed. The Pyro starts with 200 ammo, and it slowly goes down until it hits 100.
The Engineer can only lay a Level one Sentry, and a Level one Dispenser. They cannot be upgraded, the Dispenser provides no ammo or health (use it as a barricade) and the sentry is on a time limit. Its ammo slowly goes down and when it hits 0, it blows up.
Some maps are changed, such as one on Dustbowl where the Reds have to get from Stage one Spawn, in the Mines, to the Rocket in the last stage. And another has you getting to a Train on the other side of a Canyon. When the timelimit is reached, the train leaves, and if a Red is on it, Red wins.