Trump in 2020; seriously, nobody. Pence in 2024, Kamala Harris (she'd be worse than BHO, not only is she of mixed race but her husband is white. She'd really have to prove that she was blacker than everyone else), Eric Garcetti (dynastic roots still count in CA politics), Tulsi Gabbard (if she moves up to the Senate. Hirono is only 70, young for a dem senator but you never know) or most likely, someone who isn't on the radar yet.
It's hard to hold the Whitehouse for more than 8 years. A lot of things can happen. No guarantee that Pence would even run.
Chelsea Clinton?