Perhaps a magical creature that has made the island its territory. This creature doesn't want outsiders like Harry and his friends interfering. It sees Harry's glasses as a strange and foreign object and will do anything to keep them from leaving the island. It could be a giant spider - like creature that spins webs to trap intruders, just like the acromantulas in the Harry Potter series.
Well, it might be about Harry losing his glasses on an unknown island. Maybe he has to search for them while facing magical creatures and challenges specific to that island. It could also involve his friends helping him out, and in the process, they discover some long - lost secrets of the island.
It could be a dark wizard who has also found his way into space. This wizard might be using the new environment to develop even more powerful and dangerous dark magic.
It could be Captain Hook. Since Harry Potter has magic, Hook might see him as a huge threat to his plans in Neverland. He would try to capture Harry and use his magic for his own evil purposes, or simply get rid of him to maintain his control over the island.
It could be a dark wizard who has some connection to the sea. Maybe he wants to stop Harry from reaching a certain destination in the sea.
Obviously, Harry Potter would be the main character as it's in his name. Hermione and Ron might also be there to help him find his lost glasses. They could use their combined knowledge and skills to explore the island.
It could be a dark wizard who also wants to get to the secrets in the lost library. This wizard might try to stop Harry from exploring the library and getting its knowledge.
Perhaps it's an old enemy from Harry's past, like Draco Malfoy who has turned to the dark side completely. He blames Harry for all his misfortunes and decides to cage him without his glasses as a form of cruel punishment.
Maybe in this fanfiction, the island is a place filled with illusions. Harry's lost glasses are not just ordinary glasses but hold some sort of key or magic. As he tries to find them, he discovers secrets about the island and its connection to his past. For example, the island might have been a place where a powerful wizard once hid something important, and Harry's glasses are somehow related to it. He has to use his wits and magic skills to overcome the illusions and find his glasses before it's too late.
Perhaps a long - lost relative of the Potter family who feels they were unjustly excluded from inheriting the manor. This relative could be jealous of Harry's status and the wealth associated with the manor. They might use dark magic to try and claim what they think is rightfully theirs, leading to a battle of magic and loyalty within the Potter family and among Harry's friends.
A new dark wizard or witch could be created specifically for this fanfiction. This character could be interested in using the Huntsman's skills for their own evil purposes. If the Huntsman refuses, they become enemies. This antagonist could have powers related to dark forest magic, something that the Huntsman is familiar with but in a more malevolent form.