Horde mode. A few shooters have a mode like this. Vary waves of enemies and types of enemies randomly, but no matter what, they get stronger and more "elite" type enemies and even mini bosses the longer it goes on. It would be simple enough to implement on your current maps in closed off areas or even pvp arenas. Provide pve players with this experience; gives them something other than the same stories and strikes and the raid. Add skulls when you increase difficulties, too, and it will make it so that not only do different waves make it unique, but so too will the skulls (difficulty modifiers like lightswitch, juggler, etc). Finally, make it any amount of players 1-6. Gear chances or drops affected by how long you make it. Tl;dr Horde mode type game mode, loot based on length of survival. 1-6 players. Multiple difficulties.