I'll start.
SCENARIO: Donald Trump, Ted Cruz, and Marco Rubio fight out the Republican nomination all Spring with no clear winner. In the end they are split 40-30-30%, and for the first time since 1948 there is a brokered Republican convention. Determined to deny the nomination to Trump, the party is unable to choose between Cruz and Rubio, each with his own fatal flaw. Instead they go with a tried and true brand, giving the nomination to... Mitt Romney.
It makes perfect sense. The convention is in late July, leaving just over three months to ramp up a presidential campaign. Name recognition will be vital. They don't have time to introduce someone new to the American electorate. It has to be a known quantity. So who does that leave?
Other than Romney there's John McCain, but he's 79 years old and not exactly the most popular guy in the party. There's really no one with sufficient stature. It would have to be Romney.
Trump, meanwhile, enraged at being cheated out of the Republican nomination that is rightfully his, vows to run as an independent. Instead of going through the messy and time-consuming process of getting his name on all fifty state ballots, however, Trump stages an unprecedented, media-driven WRITE-IN campaign for president. Given his celebrity and his media savvy, Trump easily polls well enough to muscle his way into the debates. Once there he spends ninety minutes verbally skull-fucking Mitt Romney, poster boy for the Republican establishment. Trump reminds the audience on several occasions how much more money he has than Mitt Romney. He might even say something about the magic underwear.
The popular vote tally is Clinton 50%, Trump 30%, Romney 20%. It is an electoral landslide for Clinton, however, as the three-way race causes her to win certain states, such as Texas, that a Democrat would never normally win. Trump will claim to be the "real" victor, winning more votes as a write-in than the Republican nominee. "Biggest write-in campaign in history. Yuge. YUGE."