The only ones are the doctor and engineer.
You should throw in a few new powers - ones that enable role swaps, zombie outbreak perhaps, the lovers etc. Basically you need to build in a series of games within the game I think. I've not played it much at all, but it seems there's a lot to be said for having a number of people needing totally different outcomes in order to "win". Sometimes to the detriment of the "main" game and sometimes just with an aim totally different to the main game.