It will depend on the amount of money a candidate can expect to raise. No doubt some are already intimidated by Hillary's war chest. As should be expected, I don't see the GOP having any focus (deciding who to promote) until well into the primaries. I do know that Senators are notoriously unsuccessful.
I think using McCain would be a shame, because he doesn't carry a real Republican message. Neither do a number of the other prospects. A moderate is more likely to be successful, but give me a break here with some of these folks.