The fanfiction could be about Harry turning the island into a sanctuary for magical creatures. He might find some endangered or mistreated magical creatures and bring them to the island to live in peace. He'd then have to deal with the challenges of taking care of them all, like finding food sources and creating habitats suitable for each species.
There could be some of the house - elves that Harry has freed. They might choose to live on the island, using their magic to help with various tasks such as farming magical plants or taking care of the island's infrastructure. Also, some of the students from Hogwarts who were inspired by Harry could be there. They might be learning new forms of magic that are specific to the island or studying the unique magical ecosystem there.
Dumbledore could be a character in the fanfiction. Maybe he knew about the island all along and left it to Harry in his will. Or he could be a guiding spirit for Harry as he figures out what to do with the island.
He might have discovered the island during one of his adventures and claimed it as his own. Since there was no one else on the island at the time, and he was the first to set foot on it and recognize its potential, he decided to make it his property.
Perhaps he uses the island as a secret training ground. He could practice advanced spells away from the prying eyes of Hogwarts faculty and students. Harry might also build his own magical fortress on the island for protection and privacy.
Since the concept of 'harry owns a brothel' is against moral and ethical norms, I cannot promote or provide details about it. There are countless other great fanfictions out there that are more worthy of our attention.
Perhaps it's an island with a hidden wizarding school, similar to Hogwarts but with its own distinct features. There might be ancient spells protecting the island, and the architecture could be a blend of different magical styles. Harry could have discovered this island during one of his adventures and decided to make it his own. He might use it as a sanctuary for those in need or as a place to study and preserve rare magic.
One plot could be that Harry has to defend his ownership of the Luna fanfiction against those who want to claim it for themselves. This could lead to a series of adventures and magical battles within the fanfiction world.
One possible plot could be that Harry, after defeating Voldemort, decides he wants to do something different. He uses his Triwizard Tournament winnings to start a broomstick - making business. He teams up with some of the best broomstick makers in the wizarding world and creates a new line of broomsticks that are faster and more maneuverable. However, he faces competition from established broomstick companies who try to undermine his business.
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.