Obviously, Harry would be one of the main characters. There could also be local Harlem residents like a street - smart kid who shows Harry around. A jazz singer could be another character, perhaps someone with a mysterious past that Harry gets involved with.
The plot could involve Harry going to Harlem in search of something specific, perhaps a magical artifact if we're adding a bit of magic to the fanfic. Along the way, he meets interesting characters who help or hinder his search. He could also experience the different food and art in Harlem, which changes his perspective on life.
Well, Harry could be really confused at first. Ouran is a very different world from Hogwarts. He might be shocked by the opulence of Ouran Academy and the strange club system there. Maybe he'd try to find a way to fit in while still being true to his wizard self.
There's a chance he'll run into Ron. Maybe Ron is looking for some new wizarding sweets at Honeydukes, and they end up sharing some of the treats while they continue their shopping spree. And it's possible that they'll see Neville there too, who is searching for some rare plant seeds for his Herbology studies.
Well, it could be many reasons. Maybe there was a misunderstanding. Some dark magic might have been used to make his friends think he did something bad. Or perhaps a powerful enemy framed him by planting false evidence. It's hard to say exactly without more context from the fanfiction.
In this fanfiction, Harry might meet students from various backgrounds. There could be students from far - off magical lands who have their own unique cultures and magic. He could also run into some Beubaxtons alumni who come back to visit and they might share some interesting stories or give him some advice on how to make the most of his time at Beubaxtons.
He could meet his future self. This would be a very interesting encounter as he could learn from his future self about what he has accomplished and what mistakes he should avoid. He might also meet the future generations of his family, like his grandchildren or great - grandchildren.
He could meet the ghosts of his ancestors. These ghosts might offer him some advice or reveal some family secrets that could help him in his future adventures.