In this fanfiction, perhaps they could have an epic battle against a common enemy. Jotaro with his Star Platinum and Jonathan with his Hamon could combine their powers in a unique way.
DIO could be a likely villain. Since he has a long - standing feud with the Joestar family. He might try to take advantage of the situation where Jotaro and Jonathan meet to cause chaos and try to defeat them both.
Well, they might have an epic showdown at first. Jotaro with his Star Platinum and McCree with his Peacekeeper. But then they could realize they have a common enemy and team up. Maybe they'd exchange stories about their different worlds.
A plot twist might be that Jotaro loses his Stand abilities temporarily and McCree has to protect him. Or perhaps they find out that there's a traitor among the people they thought were their allies in this fanfiction world. This would force them to re - evaluate their situation and who they can trust. Also, it could be that a powerful artifact they were supposed to protect actually has the power to control them, and they have to fight against its influence.
One of the descendants could be Giorno. Giorno has his Golden Experience Stand and is quite different from Jonathan in many ways. He's part of the new generation of the Joestar family. Also, Josuke could be among those he meets. Josuke is known for his Crazy Diamond Stand and his strong sense of justice, which he may have inherited in part from Jonathan.
Well, in this fanfiction, perhaps Jonathan discovers amazing things about his descendants. Maybe he finds that they have carried on his good qualities or have developed unique abilities inspired by his legacy.
In this fanfiction, Jonathan is the central figure. His descendants, which could include a variety of individuals. There might be an older descendant who has preserved a lot of family history and is eager to share it with Jonathan. And there could also be a young, curious descendant who asks Jonathan a million questions about the past.
He could meet his past allies in a different context. For instance, he might encounter Kakyoin or Polnareff when they were just starting their journey. Maybe he could also run into Dio at a time when Dio was still plotting his rise to power, which would create a very tense situation.
In fanfiction, when Jonathan Joestar meets his descendants, it could be a very emotional encounter. He might be proud to see how they've carried on his legacy, perhaps in the way they fight or their sense of justice. His descendants could also learn a great deal from him directly, like combat skills or the true meaning of honor.