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.
Warren in the short term, AOC in the long term.
This seems weird, since they're separated by so many election cycles, but Elizabeth Warren is only one year younger than either Al Gore or Howard Dean.
I think there will be a gap between Warren and AOC to be fair. But there probably won't be a "successor" to the Bernie mantle between them. Unless some firebrand comes out of left field I think most radlibs will just do lesser evilism with a normal Democrat.
There will certainly be progressives but none that become the progressive. AOC could be replaced by most members of the The Squad but I think she's shown she's willing to play the game.