Another possibility is that a mysterious character in the fanfiction world, who has a soft spot for the girl, finds her lost glasses. He then uses this as a clue to track down where she has been taken. Along the way, he discovers more about the girl's situation and the reasons behind her kidnapping.
Perhaps the lost glasses are not ordinary glasses at all. They could be from a different world or dimension. Felicity's kidnapping might be part of a larger plot to retrieve the glasses for some dark purpose. The fanfiction could explore how Felicity uses her wits and skills to outsmart the kidnappers and either keep the glasses safe or use them to her advantage. It could also have elements of mystery and suspense as the true nature of the glasses is gradually revealed.
Perhaps in the fanfiction, Harry is kidnapped by a powerful magical creature that has a vendetta against wizards. When it grabs Harry, his glasses are knocked off. The creature takes him to its underground den. Harry, without his glasses, is at a great disadvantage as he can't see well. But he remembers the magic within him. He tries to communicate with the creature using Parseltongue (if it's a snake - like creature). He hopes to either convince it to let him go or at least find his glasses so he can better defend himself and find a way back to Hogwarts.
It could be a wild adventure with Harry Potter dealing with both the kidnapping and the search for his glasses. Maybe there are unexpected plot twists and magical challenges along the way.
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.
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 group of rogue Death Eaters who think that kidnapping Harry would bring them back into Voldemort's good graces. They might take his glasses as a way to humiliate him, seeing Harry as the symbol of their defeat. They underestimate Harry's resourcefulness even without his glasses.
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.
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.