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.
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.
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 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.
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.
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.
Dumbledore, if he were still alive in the fanfiction, could offer his wisdom. He might guide Harry on the moral and magical implications of creating an island. He could also provide some powerful spells or artifacts that would aid in the construction process.
Some pure - blood supremacist wizards could be antagonists. They don't like the idea of Harry creating a haven for all kinds of wizards on the island. They might try to sabotage the island's creation or attack it once it's made, fearing that it will disrupt the 'purity' of the wizarding bloodlines.
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.
In this fanfic, perhaps Harry could turn the island into a magical sanctuary for endangered magical creatures. He might build a school there for young witches and wizards who are overlooked by the mainstream magical education system.
Hermione could be one. She might follow Harry to the island to help him out. Another could be Ron, his loyal friend. Or it could be some new characters created specifically for this fanfiction, like a wise old wizard who has been living on the island for centuries.