No clue at this stage to be frank, I can't think of anyone I would be truly excited for. The Dems are hard-pressed for worthwhile young stars.
I say all the focus should be on 2018 and state races and then we'll see what's going on with the presidential candidate discussion in 2 years.

give shyt time to shake out some.
its WAY too early to call what kinda move they need to make
because if the country does relatively decent, trump or pence is gonna win in 2020. and it doesnt even have to do that well, most white voters can (and will) even delude themselves into "this is better than obama"

in which case they should just throw up a far left sacrificial lamb. maybe goofy liz warren, or franken. or hell, maybe biden

because i gotta be honest, i never saw him winning shyt with this fukkin "uncle joe" persona he has
now the flipside is if BiglyCare flops, the taxes are a sham, and the repubs fukk their voters enough to anger (enough of) them into switching. in which case i think the dems should get behind a successful governor or senator who isnt on the national radar with baggage, but can present themselves as a progressive, fresh start