The typical third in line is the Speaker, whether or not this actually happens we could potentially face a much stronger adversary, Nancy Pelosci. She may not have the charisma or the appeal of Obama to the general population but she has the political clout in congress and she's much more ambitious. At this point there really are no good options. A second election would be a fubar because who has the time/money/patience for another campaign, hell I'm going crazy from hearing about my state's damn runoff elections.
We certainly are living in those "interesting times".