This role will be neutral/benign with his goal being a successful visit to each player living at the end of the game, and also to stay alive himself.
The thief will be chiefly a mind game role. Their night ability will be stealing from each person they visit. In the case of other roles with limited ability uses, their count will go down by one if visited by the Thief. This will obviously be noticed by better players, so there should also be an option to toggle weather or not the Thief's target is informed of this or not during the night sequence. The thief must survive to the end and has no alliance except to their self and of course survivors. The challenge in being a thief and winning is to visit all the surviving players before the game ends, obviously, and not draw attention to their self by visiting targets Lookouts are watching or when under surveillance by a Detective. Thieves could be paired up with Spies or Lookouts for Investigator/Consigliere "clues."
I personally would like to see more mind game neutral roles like the Jester, Witch, Executioner, and even in some cases Survivor. This role would be tons of fun in my opinion. Let's see if DR agrees.