That depends a lot on who Romney wants to be. If he returns to being a relative moderate then someone who satisfies Tea Party types (e.g. Rubio) might be a good choice. If he is going to stay to the right than a moderate would be useful to try and pull Independent votes. Once upon a time I thought Huntsman would make an excellent VP. He would also help Romney regarding foreign policy, however, I just don't see a two Mormon ticket being acceptable to the whacky right. A two Mormon ticket is one of the signs of the Apocalypse isn't it? Before the locusts and after the seas turning to blood or something?
I really think to have a chance he will need some help with the female vote, but honestly, I can't think of any potential VP choice that helps with that.
I suspect they will be looking to avoid a Palin repeat (i.e. Vanilla ice cream covered with nuts), and so won't pick someone who is too interesting, too unknown, or too controversial. Will probably be an older, boringish, white dude (two scoops of Vanilla). In which case the most they may be looking for the VP choice to do is pull a swing state. Rob Portman from Ohio would be an example of this type of choice.