Another theme is the alteration of relationships. In these fanfictions, the relationships between the JJBA characters and the reader can be very different from the original. It could be a romantic relationship that was not present in the original series, or a friendship that forms in a very unexpected way due to the changes in the alternate universe.
One great 'jjba x reader alternate universe fanfiction' is 'A Different World of Stands'. It takes the reader into an alternate universe where the Stand abilities are used in unique and unexpected ways. The relationship between the main character and the jjba characters is really well - developed, making it an exciting read.
Perhaps he meets a version of his parents who are still alive. This would be an incredibly emotional encounter for Harry. He would get to know them, and they could teach him things about magic and life that he never had the chance to learn before.
It gives scientists new ideas for theories. For example, the idea of different spacetime geometries in alternate universes can inspire physicists to think about new models of our own universe.
One idea could be a castle that floats in the sky in the alternate universe. It could be made of some kind of magical crystal that gives it the ability to levitate. The inhabitants of this castle might be a race of winged beings who use the castle as a base to protect their realm from dark forces.
He might meet different versions of his friends. For example, a Hermione who is a powerful sorceress ruling a magical kingdom. It would be exciting as he could learn new magic from her and be part of her adventures in this alternate reality.
Well, at first, the characters would be in shock. But then, Ruby would step up. She's adaptable, so she'd start learning about the new rules of combat, the types of Grimm or other threats in this alternate universe. Yang would be a bit more brash. She might get into some fights to test the waters. Weiss would be analyzing everything from the political systems to the economic structures. She'd try to find a place where she could use her family's influence (if there's an equivalent in the new world). Blake would be on the lookout for any signs of oppression, especially if the Faunus situation is different. She'd also be the one to make sure the group doesn't do anything to offend the local cultures. All in all, they'd have to learn new fighting styles, new languages, and new ways of interacting with others.
The JJBA manga is indeed finished. It had a great journey and wrapped up with a satisfying conclusion for fans. However, there's always the possibility of spin-offs or related works in the future.