He might encounter some strange magical creatures that are only found underground. For example, there could be a new type of goblin or a magical mole with special powers.
There's a chance he'll meet other outcast wizards. These wizards might have been shunned by the wizarding world above ground. They could be very different from the wizards Harry is familiar with, perhaps having their own unique styles of magic. They might initially be hostile to Harry, seeing him as an outsider from the 'normal' wizarding world, but later they could become his allies. They might also know secrets about the underground world that could help Harry in his future battles or quests.
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.
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.
Hermione would be a likely companion. She loves learning and exploring, so she'd be eager to go on a traveling adventure with Harry. Ron might also tag along, especially if there's a chance of food and adventure. Neville could be another option. He has grown a lot in courage and his knowledge of plants could come in handy in different magical environments. Luna Lovegood would be a very interesting companion too. Her unique perspective on magic and the world would add a lot of fun and mystery to the journey.
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.
One interesting story could be about a hidden magical society within the underground of Hogwarts. They might have their own set of rules and magic that is different from what is taught above ground. For example, they could be using a more ancient form of magic that has been forgotten by most wizards on the surface.
One of the common tropes is the 'soul - bond' trope. In fanfiction, characters might be magically bonded at a soul level, which affects their relationship and actions. For example, Harry and Hermione could be soul - bonded, and it would change how they interact with each other and other characters. Another trope is the 'secret identity' trope. A character might have a secret identity or past that is slowly revealed throughout the fanfiction, adding drama and mystery. The 'reincarnation' trope is also seen sometimes, where a character is the reincarnation of someone from the past in the wizarding world.