The main character would obviously be the girl without glasses. There could also be the person who dunks her, whether it's a friend, a rival, or just an accidental bystander. Another possible main character could be someone who witnesses the event and then becomes involved in the girl's story afterwards, like a lifeguard who has to deal with the situation.
The girl without glasses is key. She could be a shy girl who is at the pool against her will, dragged there by her more outgoing friends. Then there's the character who dunks her, which could be a popular kid at the pool trying to show off. There might also be an older character, like a pool manager, who has to step in and sort out the situation after the dunking, and this manager could have a backstory that somehow ties into the girl's experience at the pool.
Perhaps the girl without glasses is near the pool, and someone accidentally bumps into her, causing her to be dunked. This event might be a turning point in the fanfiction. It could lead to her meeting someone new at the pool, like a boy who helps her out and they start a new friendship or something more romantic as the story unfolds.
Maybe it's a tomboyish girl who doesn't care about losing her glasses in the pool. She's more interested in having fun and being active in the water.
Well, perhaps she jumps in the pool without her glasses because she's in a rush to join her friends. Once in the water, she realizes she can't see clearly but still has a great time playing water games. Later, she might worry about how she'll find her glasses when she gets out.
The girl without glasses is the central figure. There may be an antagonist like a school bully who constantly teases her about her wedgies and her glasses - free look. Then there could be a wise old janitor who gives her advice on how to deal with the bullies and how to embrace her new self. There might also be a group of girls who initially exclude her but later come to accept her as she shows her true character despite the physical changes.
One of the main characters would be the girl. There could also be a pilot who survived the crash. The pilot, despite being injured, tries to use his knowledge to make the situation better. And there might be a journalist among the survivors who starts documenting the whole experience, which includes the girl's story of surviving without her glasses.
Perhaps the girl without glasses has a special ability that she's not fully aware of. The kidnappers, who are part of some sort of secret organization, know about this ability. They kidnap her to study her or use her power for their own purposes. As she's held captive, she starts to discover more about this ability herself and tries to use it to escape.
One possible fanfiction could be about a girl who usually wears glasses for a shy and studious look. When she gets drunk at a party without her glasses, she becomes more carefree and bold. She might start dancing without inhibition, surprising her friends who are used to her more reserved self.
The main character could be an aspiring actress. She's at an audition and has forgotten her glasses. In the crowd of other auditionees, she stands out because of her natural beauty and her nervous energy. A casting director notices her and decides to give her a chance, not just because of her looks but also because of the unique way she presents herself without her glasses.
She could be a young detective in a mystery - based fanfiction. Usually, she uses her glasses to carefully observe clues. But without them, she glares in annoyance when she misses something important. She might be on the case of a stolen artifact or solving a murder mystery, and her lack of glasses is a hindrance that makes her more irritable.
In this fanfiction, the girl without glasses could have a special ability that kicks in during the house fire. For example, she might have an enhanced sense of smell that helps her navigate through the thick smoke. Or perhaps she has a pet that comes to her rescue. The pet could be leading her towards the exit, using its own instincts. This would add an interesting twist to the story.