As for the hypothetical 'super-observer'? How does that player win? How do you make the game challenging for them when they know everything?
I was thinking the role would be like that of a lunatic that knows the world is wrong/isn't real, but can't convince anyone of it in flavor. And they would otherwise be vanilla town. And they wouldn't know everything, I was a bit unclear on that point. They would instead know the correct and changing rules of the game, and nothing about the players themselves. They know exactly how many mafia/town/etc are in the game, how many roles are in play, and what those roles do, but not who has them. It's a clear check on the bastard mod, as it adds some accountability. An attempt to rehabilitate otherwise bastard ideas.
Speaking of Cult balance in the other direction, maybe one player should have a role where they can find out how many cultists are in the game.
To elaborate further on my hypothetical cult with voluntary recruitment, they also can win with town if town votes to coexist. Yet there is a third faction the inquisition that has the win condition of killing all cult, with the additional requirement of killing all town if town votes to coexist with cult. A medieval witch hunter game. The witches don't have to be anti-town, although they have a night kill. They recruit by tagging town as witch initiates. Witch initiates can chose to either join the witches or remain town. But every turn they remain witch initiates, they'll show as team witch when inquisition investigates them.
3 witches, 2 inquisition, unknown town. Witches recruit or kill, but not both on any given turn. Both inquisition cops, limited number of day kills. Inquisition switches to night kills (one per night) if town votes to accept the witches.
Still work in progress, plenty of time since there seems to be room in the queue.