Pretty much anyone. Trump has already made an idiot of himself so many times in 3 months, and seems to not be interested in unifying the country, further dividing it with shit talking on Twitter, and attempted backwards laws (that have thankfully failed) that will take away more from the average citizen both in terms of rights and finance.
Not even the uneducated rural voters can ignore his idiocy for too long. By the time 2020 has rolled around, either Pence will be the new president or Trump is going to get a humility check. I expect a fairly moderate yet relatively unknown democrat.