I'd go with Liz Warren. I think she's fantastic.
A Hillary-Trump rematch would be good.
Or in a grand gesture ... Michelle Obama.
An Obama-Harris ticket would be ridiculously historic, and Trump would probably win against an all-Black female ticket.
Hill's negatives are still ridiculous. Only reason I did not name her. And your conclusion on Michelle aligned with mine.
What I went for was somebody at least as far left as Biden, with a strong positive rating. We have no politicians with the combination of popularity and name recognition to work for an incredibly short campaign - the fight to define them would be vicious. And Oprah would bring some of the same problems that Michelle would, though not all.