He might rent out parts of the building to other wizards and witches who are in need of a place to live or work. This way, he can earn some money (even though he has a lot from his adventures already) and also help out the wizarding community. He could also use the building to host magical events, like a yearly wizarding fair or a celebration of the end of the war against Voldemort.
He could turn it into a training ground for young wizards and witches. Teach them how to defend themselves against the dark arts, just like how he was taught at Hogwarts.
In the fanfiction, he might own a building that is actually a magical library. Since Harry has always been interested in knowledge, this building could be full of rare and ancient magic books. It could also be a place where wizards and witches from all over come to study and research, and Harry, as the owner, would be in charge of protecting these precious knowledge resources.
He could transform the building into a ninja academy annex. Naruto loves teaching and passing on his knowledge. In this annex, he could offer advanced ninja training, focusing on things he has learned from his own experiences, like dealing with powerful enemies and using the power of friendship and determination.
I'm not sure who 'he' specifically is without more context. It could be a fan, a collector, or someone with a particular interest in the Harry Potter fandom who has managed to acquire a collection of fanfiction.
It could mean he has the rights to certain fan - created stories related to Harry Potter. Maybe he's a collector of unique fanfiction works, or perhaps he's in a position to manage or curate a collection of such writings.
Perhaps he'd own a company that specializes in magical artifacts. He could discover new uses for old artifacts or even create new ones. For example, he could develop a new type of invisibility cloak that has additional features like being able to change colors to blend in with different environments. His knowledge of magic and his adventures would give him great insights into what wizards really need in terms of artifacts.
One possible plot could be that after the war, the Ministry of Magic is in shambles. Harry Potter, being a hero, is given Azkaban as a form of trust. He then discovers that there are dark forces still lurking within the prison, and he has to fight them off while also trying to reform the prison system.
Maybe Harry uses magic to create a unique zoo. He could have magical creatures from the wizarding world, like hippogriffs and unicorns. The zoo would be a place for both wizards and muggles to learn about these amazing beings.
Well, if he owns Harry Potter slash fanfiction, it might imply that he is in control of a certain body of work within the realm of fan - made stories about Harry Potter. Slash fanfiction is a specific type of fan - created content. He could be the author who wrote it and thus owns the intellectual property of his own creation. Or perhaps he has acquired the rights to distribute or manage this particular fanfiction from the original creator.
He could go on adventures to discover hidden vaults within Gringotts. Maybe there are vaults that have been lost for centuries, filled with ancient and powerful magical artifacts. Harry could search for these vaults, facing various traps and puzzles along the way, similar to the challenges he faced in the Philosopher's Stone.
Well, it could be a symbol of protection. In the wizarding world, Harry has faced many dangers. Owning a shield might represent his need to defend himself against the Dark Arts and his enemies, like Voldemort. Maybe it gives him an extra layer of security during battles or when exploring dangerous places.