I have a crazy prediction that probably won't come true, but I want to record it here in case it does, so you can all say, "WHAAAAAA?!?!?!" and bow to my awesome powers of prognostication.
Maybe Donald Trump in the Republican nominee, but I think this still works if he isn't. You'll end up with a Republican that nobody likes leading in the polls in October. At this point, Barack Obama orchestrates/uses a late-October emergency to "delay" the election, like how New York's mayoral election was postponed on September 11th. But there's no provision in the Constitution for this. If the states do not choose their electors in popular referenda on November 8th, and those states do not already have provisions in place for choosing their electors some other way, the joint session of Congress cannot certify a winner, and so the election of the president falls to the House of Representatives. Although it would depend on the composition of the yet-to-be-elected Congress, the current House is Republican controlled and a Republican leading in the late-October polls would give no reason to assume that would change. But the Republican Party is deeply divided, and no leading candidate is a consensus pick for the disparate wings of the party. Where would Republicans find a non-threatening competent candidate on whom they can all agree?
And this is how Mitt Romney becomes the 45th President of the United States while sitting out the election of 2016.
Yes, probably crazy enough that it will never, ever happen. Except if it does, no one in the WORLD would believe me when I said, "I predicted this last year." So now I have this blog post as my time-stamped proof.