I doubt we see impeachment. Even if Democrats take the house, which is well within the realm of possibility, them taking the Senate is almost certainly not going to happen.
What I think will ultimately hamstring him is the same thing that frankly bedeviled Obama: Underemployment and the slow death of the middle class in the economy. While the economy is very slowly improving right now and Trump hasn't really been messing with it (for better or for worse), Congressional Republicans on the other hand seem pretty keen on corporation-friendly policies that encourage growth, which aren't really necessary when the unemployment rate is low and wages are also low. If they are successful, and all available evidence says Trump will sign literally anything they put on his desk, it could just widen economic inequality.
In the words of Carville: It's the economy, stupid. You can say you're a busy president all you want (41 bills that consist of like...20 one-page congressional reviews is not 'busy'), but people are gonna know if you're just full of shit if their outlook doesn't get better.
But on top of that, what I kinda figured would happen--in terms of any legislative agenda, anyways--has happened. He can't help himself, he's constantly getting in fights, and it's paralyzed his administration from doing anything big. As much as people are frustrated that his supporters continue to hang on, sitting in the high 30s and low 40s this early in his term with an alright economy is pretty unusual.
Also, I can't help but note that he seems either unable or unwilling to read polls and understand when Congress is passing something deeply unpopular. That's going to hurt him, especially if this upcoming tax cut features explicit cuts for the rich at the expense of the poor (As the health care bill did at one point). That hurt the bill a LOT last go round, and I can see it happening again with his sign off. Eventually, he's not going to be able to be like "LOL, I DON'T KNOW WHAT I'M DOING, GIVE ME A PASS", and it's gonna fuck him. Even a lot of Republicans seem to be wearying of conservative economics, and it seems to be all Congress wants to do.
Lastly, he's run a deeply partisan agenda after losing the popular vote, and that hasn't exactly helped. Saying "Well, the people don't seem to like my policies, let's just charge ahead on them as hard as I can anyways" is a poor idea, and the only policies that poll as being as unpopular as his are a lot of the shit Republicans in Congress seem keen on doing. Resistance has hardened against him, independents are bleeding away, and he's making no effort to bring new groups into the tent. If you're refusing to make friends, you're only ever going to make enemies.
So I think he'll serve one term, barring something from Mueller. It's possible he gets re-elected; right now, I'd give it about a 50-50 shot. But I think he's going to be a Carter: fun for his fans while he's raging against the elites, less fun when satellites are falling from the sky and everything's just going to shit. He either goes along with Republicans and exacerbates the income inequality that I think screwed over Clinton's chances, or he keeps on being Trump: the administration spends four years fighting needless battles while ultimately getting nothing done. If either of those is the way it goes, which I suspect it will be, his re-election outlook will not look incredible. Especially if the Democrats find an analogue to Teddy Roosevelt, who I think is probably the kind of president we need most right now--at least in terms of what consequences he had for the economy and the political system.