I'm also thinking of a non-mafia game concept that I feel like running. It's a bit roleplay-ey and needs the player to know what they're wanting and wishing for... and I'll also draw the final result (!).
Hmm... that's an intriguing idea. I think this involves balancing player agency and the rigidity of your plot. If, say, everyone was one of two roles, then it would be easier to control the events of your story.
You could also have the story with chunks of events tied together by player actions. Maybe some people died off, but the events would stay largely the same... it's something worth looking into as a way of taking the workload off of GMs during the game.
I usually wing it when I'm a GM... hence all the crazy plot points. Case in point, Office :P
Hmm, I wonder if it's theoretically possible to run a mafia game that has a fixed progression and story (so no matter what, same things always happen), but the winning one/side can be different due to game play...
Kind of depends on how many mafia are left. Endgame, in my mind, is when either side is in a position to win during a particular phase. If there's only one mafia left, then I'd say you're in the endgame.