How about Mittens. He may be an R but he votes like a D.
It would show good bipartisanship.
I would not put money on any one of them, but if I had to name one person, I’d say Jon Huntsman. More liberal than even Romney and would not let the ‘Utah Republican” governor name a replacement. Picking Romney might mean someone closer to a real conservative might enter the Senate and decrease the progressive votes.