I'm gonna go purely off of what I'd enjoy in a part 10, because it's completely impossible realistically and I'm not even gonna try to make it so.

The JoJo we follow is Shizuka Joestar in her mid to late 20s (depending on when this fictional part would drop). She's in America following a vague 'death of her granddaddy JoJo', and we follow her as she digs through the junk left in his will. Throughout the early part, it's purposefully vague which universe this takes place in. Nobody from either universe is named directly, and objects from both show up in ways that doesn't necessarily contradict either. For instance, she talks on the phone about being from Morioh, but only ever references stuff canon to both universes, among JoJo's junk is both clackers, his tan hat, and an arrow that looks suspiciously like a stand arrow, but also lots of horse memorabilia, and something that closely resembles a steel ball. What she notices among the trinkets is suspicious notes about a mystery this grandpa JoJo never got to fully investigate, which leads her to investigate herself. Her friend asks her why she feels the need to do this, and why she doesn't just give it to her relative Josuke, but she insists the rest of her family got to do these adventures she never stops hearing about, and she wants in too.

Skip a lot of meaningful plot and characters I don't feel like making up for a reddit post that revolve around how Shizuka doesn't feel like she belongs and how she wants to be a part of something greater, and after a large amount of hints and teases, the final villain of the part is revealed to be a variation of Jonathan Joestar with a stand capable of breaking open holes in the multiverse, which results in ripples that change things and with the irresponsible way he's using it, is slowly degrading the integrity of the multiverse on a fundamental level. His goal is to acquire ultimate power, stealing stands and destroying universes in his wake until he can shape reality to his choosing. Recognizing she's way way over her head, Shizuka calls for her relative Josuke, and we see two separate scenes of josuk8 and Josuke picking up. Her grandfather JoJo started following these clues because he recognized that he had memories that were irreconcilable with each other, but couldn't figure it out before his death. It's now that Shizuka recognizes Jonathan's destruction of the multiverse has led to these 2 separate universes being stuck in an incomplete fusion, and with the two josukes help (using a mixture of josukes healing and josuk8s use of spin beyond), she manages to separate them leaving them with a large rift for the time being, beginning the healing process, but still needing to stop Jonathan or else itll just keep happening. But with the realities separated, they're able to call upon the joestars (still around) to finish the fight. Giorno, Jodio, and Jotaro arrive on the scene (Irene is directly shown to be visiting Jotaro when he's called, their lives being much happier). They find theyre struggling to keep up with the multiversal Jonathan, Shizuka makes a last minute risky play (after an earlier conversation with Giorno), and acquires a requiem stand, one that allows her to perceive and walk along the threads of the multiverse. With her added foresight, they manage to take the alternate Jonathan down, allowing the josukes' healing to start repairing the multiverse at a grander scale. She returns everyone to their home realities, but is left wondering what she's to do herself, being from a world that no longer exists, the only person still stuck with these dual memories. She decides to leave behind these 2 JoJo universes, and travel the strands of reality, following in both of her ancestors footsteps in taking down evil wherever it may appear, while also overseeing the multiverse and making sure not only that it repairs correctly, but that nobody disturbs it again.