I think Romney is a possibility, but he is kind of an unknown quantity. He will need to start doing a lot of national media exposure, and soon, if he wants to have a shot.
And, given Clinton's rapid rise in 1990-91, I think you also have to give some consideration to Sonny Perdue. He seems to have the important ability to bring in the blue dog Democrats.
I agree, although I wish he had run for reelection in MA to better demonstrate his (do I dare use the word) electability.