I think of the potential challengers, Elizabeth Warren was the most legit, and had the best chance to force a serious fight in the primaries.
I can't think of anyone else right now who'd be willing to jump in the ring & has some name recognition (at least among most progressives). Maybe a senator like Ron Wyden or Sherrod Brown?
state governors are overwhelming GOP right now (31 to 18, plus 1 independent).
I had to look them up... among the democrats who are governors... yikes a lot of them would probably run to the right of Hillary.
not sure where else you find a candidate; there are a handful of congressmen with some name recognition outside their state (e.g. Alan Grayson). I haven't read anything about drafting one of them to run. Among failed or former politicians, or others in office... there's Howard Dean... and Kucinich... eh... Joe Biden? I don't see Biden running to the left of Hillary.
Castro brothers are the future. By 2020/2024, i fully expect them to be the faces of the party.