First of all this isn't my idea, but I have no idea where it originated and can't give you a link, sorry.
Scientist: Town Investigative. Each night he gathers a DNA sample from a player, and compares his current sample with that of the previous night: he sees whether these two players are from the same team or not. He gains no other clue whatsoever as to who they are, he only knows whether they are in the same faction or not. Neutral roles appear unaffiliated to anyone they are compared with. He can't take a sample from himself or the Mayor, for that would make him overpowered.
I haven't put much effort to think if it would actually work, but salute to the actual owner of the idea - seemed to me like such a fun Investigative role to play as.