It'll be someone who isn't doing much right now, laying low, or behind the scenes.
Romney will get reminded that he is a Mormon every minute of his campaign. Especially if Palin is in the primaries. I don't think his used car salesman shtick is as prominent as people want it to be but who does he have backing him? He doesn't seem to have a lot of conservative groups going to bat for him. He'll do as well as he did in 2008.
I still think Palin can make it. Her Oprah appearance will make or break her, along with Going Rogue. These will be her two opportunities to get back into relevance. Plus if she has a lot of prominent neocons who take her under their wing, they can recreate her. Some conservatives will bitch but like they did with McCain, if she is the chose one, they'll all fall in line. If she flubs it, she'll end up like Bobby Jindal and I agree that this will be the end of the mavericks.
Huckabee has a shot but he doesn't carry much traction outside of the religious right. I suppose it all depends on what factions win but I can see them grimace at a Huck presidency more than a Palin one. Grass roots won't amount to much if the business interests want to go elsewhere. I think he is on the decline. Kind of a pick with the independent religious right but they'll drop him like a sack of bricks, especially if Palin gains traction.
I think it will end up being a Governor (not Rick Perry) or a Senator not yet considered. If the public option passes, then that could be an opportunity for Bill Frist to take a crack at the GOP primaries, for example. He didn't leave on bad terms and he is a medical doctor with a conservative slant, just what the GOP wants if the public option is passed.
We don't know if the teabaggers / 9-12 set will have a falling out or in-fighting like these organizations always end up having and who will be the casualties. If the economy picks up, all of the gloom and doom will look bad and end up backfiring. Which is why I am hard pressed to think any of the candidates now (Romney, Palin, Huckabee) have a serious shot in 2012.