HRC implied in her recent Kara Swisher interview that she's going to wait for the Democrats to put 2020 candidates forth until Trump destroys all of them slash they destroy each other, and then when there's no one left she'll step up to the plate and become the Democrat nominee. Again.
At this current point in time, I can't see any reason why this won't be the exact situation come 2020.
The economy usually predicts Presidential approval, but Trump has managed to secure an unprecedented (for this point in a first term) -10.8 point net approval and majority (52.9%) disapproval; if that keeps up the biggest danger to a challenger (well, aside from voter suppression), even more than the structural advantages Republicans enjoy, will be an overly brutal crowded primary.