A zombie-like survival scenario:
Players would be set to an entirely urban zone (ruins, skyscrapers, etc.), either have a large amount of 'survivors' or a small amount.
The players would be given time to spread out, possibly make defenses, hole up their own fortification, or spend their whole time running.
After their time is up the map would be filled with ghoul crazies, scavengers, and possibly a few supermutants and others.
The survivors would either have low hp, or the 'zombies' would have powerfists and other unarmed weaponry to deal damage.
Perhaps after the survivors die they would be turned into zombies?
Either set a time limit in which they have to survive over a certain amount of rounds (zombies get stronger), last man standing, or kill all zombies.
Survivors have the opportunity to group up, isolate themselves, work together... for a while, or fight each other for survival. Make it interesting by promising more items or some kind of treasured junk for less survivors alive, which would make the players more up for defection from their once most treasured race.