I hope it doesn't come to this, but here's the worst-case scenario I thought of:
1. Republicans take the House in November.
2. Republicans name Trump Speaker of the House. Because the speaker doesn't have to be a member of Congress, or so I hear.
3. The zombie base manages a coup against the President and VP.
4. Trump becomes President again.
It could happen as early as January. Long before he could be convicted of anything that would bar holding office. And he'd still be eligible to run again in two years.
You can prevent this at step 1 if you vote for Democrats in November.