Perhaps some new students from different magical cultures. As the wizarding world expands in the future in fanfiction, students from far - flung magical communities come to Hogwarts. They could bring their own unique magic and customs, and interact with the legacy of Harry Potter in interesting ways.
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.
Obviously, Harry Potter's children like James, Albus, and Lily could be main characters. They would inherit their father's courage and their mother's intelligence, and get into all sorts of adventures at Hogwarts.
Harry Potter himself would probably be a main character. After all, he's the hero of the original series and would likely be at the center of any attempt to fix the future. Hermione Granger would also be a key figure, with her intelligence and knowledge of magic.
Obviously, Harry Potter would be one of the main characters since he's the one doing the split. Hermione would probably also be involved. She might be the one to research why Harry suddenly has this strange new ability or how to control it.
Newly discovered magical families could rise to prominence. These families might have been in hiding or just not known before, but now they play a crucial role in the wizarding world of 1000 years later. Their members could be the main characters, bringing in fresh perspectives and magical abilities that were previously unheard of.
Hermione might also be a main character. Her intelligence and knowledge of magic would be invaluable in dealing with whatever the weapon in Gringotts is. She could be the one to figure out how to unlock the secrets surrounding the weapon, like deciphering ancient runes or understanding complex magical texts related to it.
Harry Potter himself would likely be a central figure. He has a history of fighting against dark forces and would probably step up to try and maintain order. Hermione would also be important as she is intelligent and could come up with plans to deal with the situation. Ron, with his loyalty and courage, would be by their side.
Hermione could be a major character. She would be really helpful with all her knowledge, maybe helping Harry with the care of the magical creatures and coming up with ways to educate the visitors. Ron might also be there, perhaps handling the more physical tasks around the zoo like building enclosures for the larger creatures.
Harry could become even more of a wise and respected figure. His experiences with Voldemort would have made him a great teacher or mentor in the future, passing on his knowledge of dark magic and how to combat it. Hermione might be more experimental with magic, using her intelligence to break new boundaries in magical research. Ron could be more confident and a great support to Harry in his new role.