What about a mutation that is kind of like room for one but different:
-All survivors start with magnums, pipe bombs and unlimited reserve pump shotguns
-Pipe bombs, bile bombs and adrenaline are the only items that spawn
-Bile bombs can be splashed on other survivors (if you can't do this, make it so boomers still spawn) (suggestion on how to make it work: make it so when the game detects a bile bomb exploding, it spawns a boomer and kills it instantly)
-No friendly fire
-You play on crescendo events with rush to the finish style (e.g. Parish finale)
-Only commons spawn, and they come from the direction you are running towards
-Zombies do no damage (if you can't do this, then ignore this rule) but as usual slow you down a lot (unless you have adrenaline)
-First to get to the finish wins