Most people like a conspiracy theory, if for no other reason than it makes them think. Here's a couple that actually make sense to me: One for each political party.
On the Republican side, the only chance to stop Donald Trump is for his opponents to join forces. Assuming Kasich and Carson drop out, Cruz and Rubio could meet and make the following deal: Cruz drops out with the promise from Rubio that when he is elected president, he will select Cruz to replace Scalia on the Supreme Court. With Cruz being qualified as a legal scholar and a Republican Senator, that actually could happen.
As for the Democrats, the FBI will recommend to Attorney General Lynch that Hillary Clinton be indicted over her handling of top secret information while Secretary of State. This will put the decision squarely in the lap of Obama. At this point, Joe Biden will come to the rescue and agree to run in Clinton's stead. This will happen because Obama is certainly no fan of Hillary, and the Democrat establishment knows full well that Sanders would have no chance in the general election.
Thus, we will get Rubio vs Biden in November.
Regardless, for anyone who is a political junkie, the next eight-plus months will be a hoot!