Trump faces the same realities as any other incumbent. He needs to be rated well on the things he can control, be fortunate enough to not have anything too bad happen out of his control, and respond well to crisis.
Some of what's under his control: steering the Republican agenda (tax cuts for the rich, deregulation, yadda yadda), following through on his own agenda, representing the US well internationally, etc.
Some of what's not under his control: the economy, the courts, crises, etc.
Considering his really bad approval rating, his inability to steer the party agenda, or to implement his own, and his position as an international laughingstock, he's not helping himself at all for reelection. But even with his incompetence, an incumbent has such a big advantage that he could win even if he doesn't perform well. The economy could remain strong, the court could hear a couple of landmark cases that go the way the base wants them to, we could get into some kind of war that is popular with Republicans, or face some kind of crisis at the right time.
The last (R) who failed was Bush, Sr, and he failed his reelection for two major reasons: read my lips, and pulling out of Iraq without killing Saddam. One was perceived as a broken campaign promise, and the other was perceived as a weak response to crisis. You can go deeper into congress and the economy and other things, but that's what resonates in people's minds, as symbolic of his failure. And we haven't gotten any less symbolic or short-sighted since '92. So I would look to these things:
The Wall
Obamacare
Jobs
Those are three big ones. So far he's 1/3 and the other 2 have been goddamn circuses of bad leadership. If he follows through on 2/3 of those to the satisfaction of the people, such that the independents who lean right and decide our elections approve, then he has good chances. He already has one major point in the bag with his base by appointing an anti-labor, highly religious Justice.
The opposition doesn't matter as much as people expect in an incumbent race. Take John Kerry. He came off as basically a vanilla guy, not exciting or disappointing. He was qualified and intelligent, but pretty much stayed in the center of the lane. He had a lot of unused swerving room as a candidate without changing the outcome. It was really a referendum on Bush by independents. And Bush was judged to be a good enough wartime leader that it wasn't worth taking a chance on another guy, and the economy was still doing alright. He is notable for getting a pass on his major centrist appeal of "humble foreign policy." Well, when 9/11 happens, that's just no longer a factor. The crisis gave him a pass on being judged for that.
And I don't think it changes much if the Democrats put up a populist like Bernie or someone with a similar buzz. Who in their right mind is going to get into a ra-ra bullshit-brand-fest with one of the most effective ra-ra bullshit-branders we've ever seen?