Voldemort is an obvious one. Given all the destruction and death he has caused, it's very likely that Harry would never forgive him. His actions led to the loss of many innocent lives, including Harry's parents.
Snape could be another. Even though in the original story there was more to Snape than met the eye, in this fanfiction, if Snape had done something truly unforgivable, like directly harming one of Harry's closest friends in a way that couldn't be undone, Harry would hold a grudge.
His friends from Hogwarts, like Ron and Hermione, are likely to help. Ron could bring his family's practical knowledge of wizarding homes and Hermione would contribute with her extensive research skills.
Hermione would surely be one. She's smart and would be great at helping with the academic side of building a school, like creating the curriculum.
Well, in such fanfiction, Harry Potter could meet different versions of Dumbledore. In one universe, Dumbledore could be a dark wizard instead of the wise mentor we know. He may also come across alternate versions of Sirius Black. Maybe in some universes, Sirius is still in Azkaban for a crime he actually committed. Additionally, he could meet different versions of Luna Lovegood, some of whom might have no interest in the strange and wonderful things she loves in the original universe.
Common characters he could meet include an au Ginny who is a powerful seer. There might be a Hagrid who is a tiny, timid man instead of the large, boisterous one we know. He could also encounter an au Sirius Black who is a Death Eater, which would be a shocking twist. Additionally, he may run into an au Professor McGonagall who is more of an artist than a Transfiguration teacher.
The doctor could also be a Muggle - born wizard who has combined Muggle medical knowledge with magic. This kind of doctor would approach Harry's situation from a different angle. They might use Muggle - like scans enhanced with magic to look for internal problems. They could offer a fresh view on how magic interacts with the body, which could be very helpful in diagnosing and treating whatever is wrong with Harry.
Harry could encounter a group of wizarding surfers. They use magic to enhance their surfing experience, like creating perfect waves or making their boards fly over the water. They introduce Harry to a more laid - back side of the wizarding world in California.
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.
In the fanfic, Harry Potter could meet the new generation of students. They would be in awe of him. He might also encounter some of the ghosts, like Nearly Headless Nick, who would surely have some interesting stories to share since Harry left. And there's a good chance he'll meet the caretaker, who would probably give him a hard time about coming back and causing a stir. Moreover, he could cross paths with some of the older students who idolize him and want to learn from his experiences.
It's possible that an old acquaintance from Hogwarts who has family ties in America could assist him. This person might already have a network in America and could introduce Harry to important people. Also, there could be some reclusive magical scholars in America who, out of curiosity about Harry's powers and his role in the UK, decide to help him. They could teach him new magical skills that are specific to the American wizarding world.
Harry could meet some local magical creatures that are unique to Africa. For example, there could be a magical creature similar to a hippogriff but with different characteristics. And he may also meet African witches who are experts in herbal magic, which they use to heal and protect their communities. They could teach Harry some of their techniques and knowledge.