I have a few ideas.
Zombie Siege - A group of players defends a castle from zombies. The zombies could have potion effects, armor, bows, but they should probably have bigger numbers and scaled to how many players there are. Kits could be pretty much stolen from KitPvP :D
Parkour Puzzle - A small group of players (depending on the map, maybe 2-4 or so?) goes through a parkour puzzle. If they succeed, each of them gets a record time, much like the Parkour server.
Boss Fight - Design a mob, with several abilities. (Such as spawning other 'minion' mobs, making an explosion, shooting fireballs, or dropping a lava block somewhere random. For this last one, I would spawn in a FallingSand entity of lava, to not surprise players.) After this is complete, either scale the health/attack power to the number of players, or set a player size, or both.