Well, maybe the question should be who wants to run for Vice President with the potential to become Vice President under John McCain. The list could be long but, really, who wants to be Vice President besides Hillary Clinton?
Mitt Romney has tried to cozy up to McCain. I'm not sure being governor of Massachusetts is a huge plus but he's known thanks to the Olympics and his unsuccessful bid for the Republican nomination. Another unsuccessful candidate could be Mike Huckabee. If McCain really wants to alienate the conservative base of the Republican party, he'll pick Huckabee. Very unlikely.
Maybe another Bush? How about Jeb Bush from Florida? Talk about dividing a nation. Bush would definitely chase a bunch of people away. The Independent Joe Lieberman is out there. I doubt Joe would want to run for Vice President again and, as before, this would definitely alienate conservatives who think of Lieberman as the Vice Presidential candidate with Al Gore.
What about Condoleeza Rice or Colin Powell? Rice is a pretty strong candidate but Powell lost favor in the party when he kinda split with the Bush Administration. A Rice pick would probably be looked at as an attempt to gain a black vote...which it most certainly wouldn't.
Most likely candidates are Mark Sanford, Governor of South Carolina; John Thune, Senator from South Dakota; or Tim Pawlenty, Governor of Minnesota. All are relatively young (compared to McCain) and are rising stars in the Republican party. The question is whether any of them are willing to leave their current posts to run on a questionable ticket with McCain.
My money is on Sanford at the moment. He didn't rush to endorse McCain but he's a strong Southern Republican and his economic conservatism could win big points with the Republican base. Of course, I'm putting my guess that McCain wants to win over conservatives. With McCain's track record, he may try to get Bill or Hillary to run with him. I'm just not sure anymore...