I am generally wary of anything that introduces a third faction or radically changes the nature of the game in the middle of it. I think it's too much uncertainty and source of confusion. Not to say I wouldn't implement something people want.
I like the idea of the tracker and the watcher (
here). Tracker can see the target of any other player's actions, but not the action. A Watcher can see who else targeted a particular person, but not the action. I thought of a similar role before called "psychic" where they see a list of people acted upon, but not who did the acting or the action. That'd be a more powerful version of the watcher.
Jailkeeper is also interesting. They protect a certain player but also invalidate that player's actions, if they have any. i.e. you protect the seer but they can't scan that night.
edit: The latter list of roles also allows for a "tree stump." Someone who cannot be lynched but also cannot vote. In some version, you can render others as tree stumps, or one can choose to stump themselves.
edit 2: If I do run a game, I would very much like to implement a tracker or watcher. Perhaps giving the selected person a choice too. Hmm...
