Perhaps in the fanfiction, Harry discovers a new spell or a magical item that can temporarily enhance his vision without glasses. But there could be a catch, like it only works for a short time or has some side - effects. This would lead to a whole adventure of him trying to find a more permanent solution while also dealing with the normal challenges in the wizarding world.
He could misidentify people. Maybe he mistakes a Slytherin for a Gryffindor and gets into an awkward situation. It would be a humorous scenario where he has to quickly correct himself.
A fanfiction idea could be that without his glasses, Harry's world becomes a bit of a blur. He starts to rely more on his other senses. In the Forbidden Forest, this could be both dangerous and enlightening. He might be more attuned to the sounds and smells of the forest, but at the same time, he could easily walk into a dangerous area. His friends, Hermione and Ron, would have to be extra vigilant in protecting him. This new dynamic could change the relationships between them and lead to new adventures.
One possibility could be that in this fanfiction, Harry's glasses are some sort of magical anchor. Without them, he might accidentally slip into a different dimension. Maybe he was in the middle of a battle at Hogwarts, his glasses got knocked off, and suddenly he vanished. It could be that the glasses were enchanted by Dumbledore long ago to keep Harry in the 'right' magical space, and without them, he's lost in a sort of limbo.
For the drama. It creates a sense of vulnerability for Harry. His glasses are an important part of his identity, and without them, he's at a disadvantage. This can heighten the tension in the story.
One possible scenario could be that he's being dragged by some dark magic force in a forbidden part of Hogwarts. Maybe he lost his glasses during a fight or while being chased, and now he's in a vulnerable state. Without his glasses, his vision is blurry, adding to the sense of danger and confusion.
One possible plot twist could be that without his glasses, Harry has a hard time recognizing his captors. It might turn out that they are people he thought he could trust, like some seemingly friendly Hufflepuffs who are actually under the Imperius Curse. This would add an element of betrayal and confusion to the story.
Another idea is that some Death Eaters find Harry without his glasses in a vulnerable state. They start by taunting him about his weakness without the glasses. Then they use dark magic to make him experience extreme pain, like a searing pain in his eyes as if they are trying to make him feel the 'punishment' for relying so much on his glasses. They might also try to force him to perform difficult magic tasks without being able to see clearly, and punish him severely when he fails.
One possible fanfiction could be set during a fight with some dark wizards in a back alley. Harry is outnumbered and gets beaten up badly. In the chaos, his glasses are knocked off and broken. He has to rely on his other senses and magic to fend off the attackers until help arrives.
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.
A fanfiction might have it that a new dark force in the wizarding world targets Harry. They manage to trap him in an old, abandoned manor. They torture him using various dark spells, and during the chaos, his glasses are lost. Harry endures the pain and tries to remember some of the spells Hermione taught him for self - defense. Eventually, he breaks free from his bindings and starts looking for his glasses. He stumbles around the manor, using his wand light to search, while also being on the lookout for the return of his captors.