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.
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.
Ron might be there too. He could use his skills in handling magical creatures to populate the island with friendly ones. And he could also help with fishing in the surrounding waters using magic, providing food for those on the island.
I'm not sure specifically as there could be many different takes in such a fanfiction. It might be about Harry Potter discovering or owning an island, perhaps with magical elements on it, like hidden magical creatures or ancient spells.
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.
Perhaps Hagrid could assist. He has a deep understanding of magical creatures and could help Harry populate the island with them. He might also use his strength to help with the physical construction, like moving large rocks or trees.
Maybe Harry uses magic to create an island as a secret hideout from the Dark forces. He could make it full of magical creatures and plants that he discovers during his adventures.
Well, perhaps Harry, tired of the wizarding world's chaos, decides to buy an island for some peace. He could use magic to build a unique, hidden mansion there. Maybe he'd invite his closest friends like Hermione and Ron to visit, and they'd have magical adventures on the island that are different from those in Hogwarts.
Well, one possibility could be that Harry discovers a hidden magical island. He might find new magical creatures there and learn unique spells to survive. Maybe he builds his own little wizarding home on the island.