There are always a lot of unknowns this far out. There are two big unknowns: 1) whom will the Democrats nominate; and 2) when will the next recession start.
It is becoming clear that the race for the Democratic nominee is basically open at this point. While Vice-President Biden appears to have the best chance to win, that chance is maybe 1 in 4 (or about the same chance that Trump had back in late October 2016 of winning the electoral college). And since somebody has to win the Democratic nomination, that actually makes him the "favorite," but it is substantially worse shape that Secretary Clinton was this time four years ago. And how the Democratic primary plays out will impact the winner's chance of winning the general election. Clinton's win about four years ago left a bad feeling (in large part fed by Russian trolls) in the taste of some progressives who opted to vote for third party candidates as a protest -- especially when it seemed like Trump had no chance. A similar divide in the Democratic Party would dramatically improve President Trump's chance at reelection.
On the other hand, we are ten years in the expansion that started under President Obama. That is an incredibly long period of time since the last recession. So the question at this point is pretty much when the next recession will start and how deep and long it will be. If, somehow, the recession does not start before summer 2020 (which is hard for economists to imagine), then Trump might be able to win if he can keep out of his own way. While the U.S. is so bitterly divided that even a recession would probably not result in a landslide win for the Democratic nominee, it would make a repeat of 2008 more likely than a repeat of 2004 or 2016.