The world of Full Throttle was fun, so I wouldn't mind seeing another game set in that world, but I don't think the second game should center around Ben and the Polecats, since I think Ben's story was wrapped up very well.
It could be interesting to see through the eyes of a rival gang. The Vultures would be a nice choice, since we only got to see them for a short time after it was revealed Maureen was a member of that gang
. For a bit of a change of pace, the game could center around the Rottwheelers, since they appeared to be much more brutal than either the Polecats or the Vultures, so the tone of the game would surely be different as a result.