My breakdown of potential candidates listed on Wikipedia (except for a few whom I've hear practically nothing about):
Democrats
No chance (may run)
Markell
Markell did float the idea of running for prez in 2016 last year, but since then he's endorsed Joe Biden for 2016 (assuming that Biden runs). So I think Markell's out as a potential candidate. (Not that he would have had much chance at the nomination if he had run.)
Interesting, I hadn't caught that. His potential candidacy baffled me: Why on earth would a rumpled and uncharismatic governor of one of the smallest states in the union think that running for president was a good idea?