Boris becomes PM but no chance of a new exit deal from the Brussels bureaucrats who all have the look of someone wearing diapers, these days, but Merkel and co finally come out with a better deal than they offered Cameron to stay, mostly based on a much stricter residence test before access to benefits, tax credits, social housing etc - that means Boris has an excuse to call a second referendum on no deal v remain. He gets that through by saying no deal will win but then decides to go neutral, let the British people decide, only here to serve, etc.
The very restrictive freedom of movement rules that mean anyone can come here but they have no access to benefits etc for the first five years takes the bite out of remaining in the EU for the hardcore Brexiteers whilst the sixty percent of the populace who want to remain will be much relieved. Boris and Farage will then have to fight it out on who wants the smaller state, lower taxes for the next GE. Labour and LibDems/Greens will also split the left vote. Boris should still be able to slam his way to power, though, especially if he offers Nigel a top job if he joins the Conservatives!