It depends on how the narrative is spun after McCain loses. If Republicans believe that Palin dragged McCain down (which appears to be an objective fact), she won't go much further.
If she and her enthusiasts can claim that the McCain campaign held her back from realizing her true potential, then she might be a rising star in the GOP. And given the Republican "base" capacity for self-delusion, that seems like a likely scenario.
I really think the time is ripe for a new party that represents traditional conservative views -- limited government, meaning not just tax cuts but limited foreign interventionism and meddling in private affairs.
For those of you Republicans out there, since McCain/Palin is a doomed ticket, you can best express this desire by voting Bob Barr on November 4.