The 2016 Republican Nominee will be certain right after 2012 is over. It's always whoever is next in line.
And who is that supposed to be? No obvious heir is present and I could easily argue that Sarah Palin and Mike Huckabee were more obvious heirs for this year. The current primary system isn't very old and we have too few datapoints to really say anything definative like that. Added to that the fact that quite a few GOP heavyweights look like they'll jump into the 2016 fight and I don't think this will hold true at all.
