Have you read Beyond Canon? Knowing some of the characters might potentially give us candidates for who could enter another Sburb session, although everything remains to be seen...

A previous player entering a new session is a really fun idea. My guess is that with how intertwined Sburb is to the continuity of reality, it would be one of those things that Paradox Space would not allow to happen in the first place, or at least create a doomed timeline if it did. But, hey, crazier things have happened, and it would be absolutely in-line with Homestuck to add a whole layer of new bullshit and end up with someone who has godtier^2ed or something like that.