I know a lot of people don't like overt team pvp style game modes, but here goes...
Game Mode: Mongol Invasion
To the surprise of everyone, a Horde of Space Mongols (10-20 players, maybe more!) invade the station. While their scimitars and recurve bows are primitive, their lightning fast steppe horses(think janicarts) and massive numbers allow them to quickly seize a large chunk of the station from the unprepared crew. Unlike other antags, which allow the crew to hunker down and wait for sec to solve things, the Mongol threat doesn't accommodate such decadent complacency, and requires the Heads to harness the crew to a strategy of Total War, lest the entire station be slaughtered by a ruthless Horde that has never known defeat.
Anyway, the idea of this game mode is to turn a common antag gameplay dynamic on its head, namely the one where the antags start off weak and gradually increase their numbers or their power (Cult, Blob, Aliens, Vampire, etc.) With this mode, the Mongols begin the round as powerful as they're ever going to be, and their initial strength will hopefully spur the crew to rapidly organize a war of attrition once the initial conquests of the Horde have been blunted. No more "Aliens on the station? Meh, weld the vents and wait for sec to kill them."
I know this game mode would get dull if oft repeated, but it could be a nice palette cleaner after boring rounds where changelings and traitors discreetly complete their objectives while oblivious crewmembers work at their tedious tasks and the detective shrugs haplessly at the Unknown corpses found in lockers. Plus it would give more players an opportunity to be an antag, which I know a lot of players want. Maybe it could work as an event instead of a game mode. Imagine a bloody revolution which is gate-crashed by the Mongols, causing the Revs and Heads to form a tentative truce until the existential threat is dealt with. This would be fitting, since the actual Mongols often turned existing political conflicts on their heads due to their astonishingly rapid conquests.
Yes, I'm talking in broad strokes here, and haven't really talked much about balance details, such as how the Mongols can bypass doors or subvert the AI, but I still think it could be a really cool game mode. Feel free to offer criticisms or suggestions!
Also, on a related note, Mongol throat singing is totally awesome. Lend your ear to this. (The best part starts at 1:49)
Now if that doesn't make you want to mount your steppe horse and start slaughtering greyshirt peasants, then what would?