I think about this a lot. But who could it be? There’s no one with name recognition who is even thinkable. Schumer? One of the one-percenters that no one has even considered like Bennet? Oprah?
Dunno.
My wild-assed guess about the Dark Horse though:
1: Will be male, handsome, eloquent and probably rich (so female voters, the most important Democrat voting bloc, can fantasize about him).
2. Will not have an extensive public record of political statements or actions. (So that Low Information Voters can pretend he supports whatever they hold important.)
3. Probably not a minority (Obama blew that out of the water for the time being).
Name recognition is, literally, a minus - it would mean that they have a public record of political or business statements.