The most obvious possibility is AOC. In that case I am sure she'll move even further right and get totally ratfucked by the party regardless. That's the most jokerified option, I think. At the very least, her obvious ratfucking will push many more people to the left like the Bernie campaigns did.
But the worst possibility is some rando Democratic apparatchik we've never heard of like Mayo Pete showing up. They take up all the aesthetics of Bernie's campaigns, they receive the glowing endorsements of Bernie and the squad, but they seemingly refuse to outright promise anything we would call "good", and they suspiciously receive no material push-back whatsoever from the Democratic party establishment other than some clearly bad faith grumblings about how spooky and radical this person is. Win or lose, they completely suck the energy out of leftist movements in the imperial core for a further decade.
AOC either runs for president or she remains in congress like Nancy Pelosi and doesn't contend any races outside of her congressional district. The DNC has been fond of putting in tokens. They could get a "random" who's fairly well known to those paying attention but unknown in the public mind. There are plenty of people in congress and the senate who can claim they're progressive for supporting a handful of policies on paper.
Pete seems to be who they're pushing. The DNC is either more out of touch than I thought or suburban boomer Democrat voters like him more than I thought.