A Democratic breakthrough that so far I think I'd like to see is Elizabeth Warren. She is sensible, she's a policy wonk, she knows her stuff.
Forget Bernie, way too far left for the United States (he'd be a better fit in Canada, though I doubt I'd vote for him). Kamala Harris is interesting and sharp, but I don't know enough yet. Biden's past it, though probably a reasonably decent man, and somewhat less frightening (for the "I hate change" types) than the rest of them.
Honestly, I think warren could do it...