Alexandria Ocasio-Cortez, Pete Buttigieg, Elizabeth Warren, Andrew Yang, plenty of good candidates.
Sanders would be a bad choice because he’s too old and while he’s certainly more compos mentis than Biden, he cannot escape that criticism. Michael Bloomberg would also be a similarly bad candidate for the same reason, plus also for running an incredibly expensive and half-assed campaign last time around.
Ladbrokes currently have Michelle Obama as the second-favourite to win the Democrat nomination. She would actually be a great choice because it means Barack gets another four to eight years in the White House, albeit as the First Man. She also seems like a very sensible candidate in her own right.
Alternatively, they could go the Reagan/Trump route and usher in a celebrity as their candidate. Someone like Oprah Winfrey, Taylor Swift, Stephen Colbert or Jimmy Fallon would be huge.