He could meet a young and brilliant wizard inventor. This inventor is working on some newfangled magical gadgets and shows Harry some prototypes, like a self - flying broomstick that can change its shape according to the rider's mood.
Perhaps he'll run into an old wizard who claims to be a friend of his parents. This wizard might share some long - lost memories or secrets about James and Lily Potter that Harry has never heard before. For example, a story about how they once helped to save Diagon Alley from a dark wizard's plot when they were still at Hogwarts.
He could meet a famous wizard author who is signing books in Diagon Alley. This wizard could give Harry some valuable advice about his future.
He might meet some shady characters like Borgin and Burkes. They deal in all sorts of strange and often dark items.
He could meet other fans of the same fictional universe. In some fanfictions, gay clubs are places where fans gather to discuss their favorite characters and storylines. So Harry might run into people who are also fans of his world.
He might meet his parents. Since they have passed away, the other side of the veil could be a place where they exist in some form.
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, he could meet some new and mysterious characters. Maybe a young wizard with a strange power who becomes his friend or an old wizard with a hidden agenda. He might also discover a new magical shop that sells unique items not found in the regular Diagon Alley stores.
Well, he might encounter a new magical creature that is not in the original books. For example, a tiny dragon that can speak and becomes his little friend, guiding him through the Alley and showing him some hidden shops.
Maybe it's a new character created by the fanfiction author. This new character could have special abilities or a unique backstory that ties into the reason for escorting Harry to Diagon Alley.
In the fanfiction, Harry could meet Native American wizards or witches who have a deep connection to the land and ancient magic. Their magic might be based on nature and spirits in a way that is very different from what Harry is used to in Britain. He may also meet some Muggle - born American wizards who have a different perspective on being part of the wizarding world, having grown up in a different cultural and social environment compared to Harry.
Well, in some fanfictions, it could be due to a powerful spell gone wrong that transfers the ownership to him. Maybe a spell meant to protect Diagon Alley's property rights backfires and suddenly Harry is the owner.