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.
I don't even think Bernie could be the successor to Bernie going forward. 2016 an 2020 did a lot to turn young Americans onto leftist (and left-ish) politics, but in the same move they definitively turned that group off from electoralism, and then the man himself drove the final nails into the coffin of his own movement by refusing to meaningfully oppose the Democratic establishment.