I was playing through the "Sweet Tour" trivia game last night and you know, it would have been really cool to go around as ST in Blackfield Asylum fighting enemies.
I also think some of the other interior locations in Harbor City were pretty fitting locations for ST to cause chaos in.
-a church:
-a strip club:
It didn't seem like Harbor City had any on-foot missions in open outdoor areas though, but more maze-like exterior locations like the parking lot in 'Sweet Tour'.
The issue I'd have with maze-like exterior locations is that it could get annoying. Either because of the Camera, or because you can get lost, and not back-track easily.
I think a SweetTooth 3D or side-scrolling Platformer could be interesting too. Instead of a big world map, just have a bunch of stages in different locations, full of enemies, traps, weapons, with SweetTooth alternating from on-foot to vehicular missions.