Hillary has some of the highest negatives on the planet.I'm thinking Hillary: she's recognizable, didn't do anything that would compromise her (more than she already was) in the past few years, a lot of people already forgot about her scandals (and many young voters never even heard about them). They can use the first woman president / women rights narrative to rally the females, and also try to sell her as an experienced statesman who'll take on Putin (conveniently ignoring the Russian-reset policy or the uranium one scandal).
There is zero chance.
The biggest issue is getting a new name on the ballot and funding.
They can't transfer any money for Biden/Harris campaign to a new campaign...only money givens directly to the DNC.