Kubo showing him in a flashback, might mean that he will join the next battle against Quincy, seeing as many Shinigami have died and they need to fill their spots.
I guess Aizen wasn't vary of Zaraki for nothing. It might have been Aizen's end if Zaraki were to return to his old self. If Aizen and Zaraki met as kids, Aizen might have chosen a different path, since he became like this because he was lonely, or something, well he was strong the moment he was born and had no one stronger to look up to (it was something like that).
I wonder if the SK had illegitimate children.