The Anti-PvE event.
Everyone present gets a mantis, ant or DesertStalker skin and KEKEKEKE ZERG RUSH! the NPC defenders of locations. They should be well-armed, in good amount and cant be respawned. In the end, the supercomputer representing 2238 server in restricted area in VC will be stormed and destroyed.
I like the idea, but no Desert Stalkers or Rad Frogs, or whatever the hell they're called (even though they have great animations), because they demolish the fps to bare minimum and the cursor starts floating like a drunkard.
Maybe half of the players can take on the "monster role" and the other half the survivors?
EDIT: There's plenty of animals to choose from: deathclaws, centaurs, floaters, aliens, cyber/dogs, geckos, radscorpions, mole rats, ants... all they need is a buff on their hp/damage/resistances to balance out the strength.