One interesting plotline could be the different magical techniques from each era being shared. For example, the older era might have some long - forgotten spells that the younger characters find fascinating and try to incorporate into their own magic. Another could be the cultural differences within the wizarding world across the three eras. Characters from different eras might have different views on things like house - elves or blood purity, leading to some intense discussions and possible changes in perspective.
Harry Potter himself is very likely to be one of the main characters. Since he's such a central figure in the series, it would make sense for him to be involved in a story where all three eras meet. Hermione Granger could also be a main character as her intelligence and knowledge of magic would be very useful in a situation like this. And perhaps Albus Dumbledore from an earlier era, as he is a very powerful and wise wizard.
Well, if we meet Uncle John in a Harry Potter fanfiction, he could also be a character from a different magical culture. He might introduce new spells, creatures or ways of magic to Harry. His presence could expand the already rich world of Harry Potter and create new adventures for Harry and his companions.
He could meet new witches and wizards from different cultures. For example, he might run into a powerful African wizard who has unique magical abilities related to nature magic. Or he could meet a Japanese witch who specializes in charm - making that is based on ancient Shinto beliefs.
He might meet international aurors. These are highly skilled wizards from different countries who are experts in dark wizard catching. They could teach Harry some new combat spells.
He could meet Fleur Delacour's family. Since Fleur is from France, her family might be an important part of the French wizarding community that Harry interacts with.
He could meet his future children, if he has any. They would likely be powerful wizards or witches with their own adventures. Also, he might run into the future versions of Hermione and Ron. They would probably be in important positions in the wizarding world, like Hermione might be a headmistress at Hogwarts or something.
He might meet the future versions of his friends like Hermione and Ron. Their children or grandchildren could be there, and they might have different personalities and magical skills compared to their predecessors.
He might run into some of his old professors like McGonagall. She would be glad to see Harry back at Hogwarts. And perhaps he'll also meet some new students who look up to him as a hero. These new students could ask Harry for advice on how to be brave in the face of danger and how to master difficult magic spells.