Yang? Already flamed out once, and had very few specific policy goals beyond UBI.
AOC? If she accepts, which I doubt, she’ll face challenges of “she’s too young” despite her being of-age by the inauguration, and would certainly see a federal or SCOTUS case trying to disqualify her. I could see BOEs in multiple states trying to boot her off the ballot because of it.
Warren and Buttigieg both are saddled with lots of baggage - Warren’s already been tarred as a nut by mainstream media, and Buttigieg’s performance as Secretary of Transportation is…lackluster.
Michelle Obama would be foolish - it would see the same challenges as Hilary in the “we’re building another dynasty” angle.
Oprah is a poor choice, Taylor Swift, same. Colbert and Fallon would seriously be BAD choices.