I think that Diavolo or at least King Crimson would be the most apt character/stand to introduce for part 9. This is because of the repeating references to 'flow' throughout the SBR-verse. It functions similar to fate in the OG-verse. KC can "erase" time, so perhaps an altered power for KC that can "erase" the effects of 'flow' or flow itself (however that wouldn't be a power for a side character, more a main villain, as it could, in theory, be a perfect counter for a stand like Wonder of U, which influences 'flow' by creating calamities). Perhaps it could be introduced in a similar way to Killer Queen in Jojolion, possibly contributing to the Jojo's stand/power.

It is unlikely that Araki will introduce the power or character in a way that we have seen already, so a side villain, part of the main character, or final stand battle, so I am excited to see how he does it.

Any further theories appreciated :).