Personally, it is hard to imagine any president resisting the will to power, the thrill of campaigning, and the chance to better chisel his policies and legacy. However, Trump came in unconventionally, and it would not be surprising if he went out the same way.
Assumedly, Pence would make a run in his stead, backed by the Koch brothers. In my opinion, Trump would do better if he were to appeal to the full range of the populist movement, part of which got him elected. It would be nice to dream of him urging Bernie Sanders to run as VP to some palatable Republican, but I don't see that as possible these days.
I think Trump could break ground by urging Nader to run with some palatable Republican as VP - or running someone like RFK Jr. as a VP. What I am saying is that it could be a winning strategy to run a libertarian-progressive coalition ticket, (WITHOUT corporatists like Kochs), considering both the success and vulnerability of his base. How to reconcile on a number of hot-button issues: Work would have to be done on that, over time.
If Bernie ran against Pence, he would stand a good chance of winning. However, the overall fight could be contentious, and, this time, it might be Hillary Clinton who ends up being the monkey-wrench. If Clinton ran, she could win, but the whole thing would just inspire more populist unrest.
Roger Stone: Trump May Not Run in 2020
Roger Stone thinks Trump will not run for re-election
Donald Trump May Quit After One Term...
Nothing is set in Stone..