I'm assuming it'll be another bout with Thompson. I doubt very much that we'll ever see UFC contenders 'move down' to SF. And let's be honest, it would certainly seem like a demotion to anyone vying for a UFC title. Not to knock SF, which I often watch, but it certainly seems like a 'feeder organization' to the UFC as of late.
Penn would be an interesting match-up, though as was said, I don't see him taking it. I think he was serious when he called it quits.