Paul Ryan
Not in 2016, but, in 2012? Why not!? As I understand it, the position of the Vice President of the USA is that if anything happens to The President, the next person in line is the Vice President. So if Romney pulls out of the race for President, technically the Vice President would more or less be guaranteed that he would be leader of the party and effectively the person looking to be President, therefore if he gets the most electoral votes then the former Vice President nominee would effectively be President!
Have a little flutter on Paul Ryan being elected President 2012!