I think Wisconsin Congressman Paul Ryan has enough youth (late 30s) and Kennedy-esque good looks to at least offset McCain's age and respond to Obama-mania.
Ryan's an Irish Catholic from a swing state. He's anti-amnesty (for real, I know they all SAY they are) and clears all the other litmus tests (guns, gay marriage, abortion, taxes, etc) so he will avoid angering/disappointing the base and in fact will excite Republican activists. Best of all, his work in Congress has focused heavily on economic issues. He's compatible with McCain because he's crusaded against pork earmarks, but his work is much broader, on entitlements, overall spending and more. He'll help address the economy, which McCain has admitted he's weak on (a statement the Dems will pounce on).
My criteria are:
1. Qualified to take the helm immediately in event of the disability of the president
2. Ideologically compatible with the party (which is, I know, highly subjective)
3. A potential standard-bearer after the McCain administration is over, or in the event of a McCain loss
Given that, the best name I've heard so far is Chris Cox. He's got extensive and varied experience both in life and in government; he's truly smart; he's reliably conservative; and he's personable enough to be President in his own right.