Well, perhaps in the fanfiction, Vernon's torture of Harry could be a way to try and force him to give up his magic completely. Harry might resist and find an unexpected ally within the Dursley household, like Dudley who has a change of heart. Or maybe Harry's friends from Hogwarts somehow sense his distress and come to his rescue, leading to a big showdown between the wizarding world and the Dursleys.
One possible plotline could be that Harry uses his magic in self - defense and this leads to a whole new set of challenges as he tries to hide it from the wizarding world while dealing with Vernon's continued abuse. Another could be that the Dursleys finally get caught by the wizarding authorities for their mistreatment of Harry, especially Vernon's torture.
In some fanfictions, Vernon's torture of Harry could be more psychological. He could be constantly demeaning Harry, trying to break his spirit. Harry, in response, might find strength in his memories of Hogwarts and his friends. He could also start to show signs of accidental magic that Vernon can't control. This could escalate the situation until Harry decides he has to leave the Dursley home for good, and he embarks on a journey to find a new place in the wizarding world while being hunted by Vernon who is afraid of the consequences of his actions being exposed.
One possible plotline could be that Harry is wrongly accused and sent to Azkaban. While there, the Dementors constantly torture him with their presence, draining his happiness and hope. Another could be that a dark wizard within Azkaban has a vendetta against Harry and uses dark magic to physically and mentally torture him. Or perhaps the prison system itself, with its harsh conditions and lack of proper care, is a form of torture for Harry, and he has to find a way to endure and escape while dealing with the psychological trauma.
Perhaps Harry is held captive in a hidden location by Voldemort. While being tortured, he has visions of his past and the sacrifices his parents made. This gives him the strength to resist Voldemort's attempts to break his will. His friends then embark on a dangerous quest to find him based on these visions Harry manages to send out magically.
One possible plotline could be Harry finding a magical way to fight back against the Dursleys' torture while still hiding his magic from the Muggle world. For example, he might use some subtle magic to make things go wrong for the Dursleys when they are being especially cruel. Another could be that his Hogwarts friends discover what's going on and come up with a plan to rescue him during the holidays. Maybe Hermione uses her knowledge of the law to find a way to legally remove Harry from the Dursleys' custody.
One possible storyline could be that Harry endures the torture but uses it as fuel to secretly plan a rebellion within Hogwarts. He might start to gather like - minded students who have also been victims of Umbridge's tyranny. Another could be that his friends, like Hermione and Ron, find out about the torture in a more indirect way and then they try to expose Umbridge's actions to the wizarding world. Or perhaps, Dumbledore finally steps in to put an end to the torture in a very dramatic and magical way.
In this kind of fanfiction, perhaps Vernon is driven by jealousy. He sees how much attention Harry gets, even if it's often negative attention within the household. This jealousy festers until he decides to take drastic action. It could also be related to some long - held family secrets or grudges that are finally brought to the surface, leading Vernon to try and kill Harry.
Some fanfictions might explore a backstory for Uncle Vernon. Perhaps he had an encounter with a wizard when he was younger that made him so bitter towards magic. In these stories, Harry could discover this secret and it could change their relationship. Also, there could be a plot where Uncle Vernon is magically turned into a wizard for a short time. This would be a comical yet thought - provoking plotline as he would have to navigate a world he once despised.
One possible plot could be that a dark wizard, perhaps an escaped Death Eater, infiltrates Hogwarts during a time of vulnerability. They use a powerful dark spell to paralyze everyone in the Great Hall except Harry. Then they start to torture Harry to try and extract information about Dumbledore's secrets or the location of the remaining Horcruxes. Another idea is that a group of rogue students, influenced by dark magic, decide to take revenge on Harry for his fame and success. They use some sort of dark charm that causes intense pain and torture Harry in the Great Hall, in full view of the petrified students and teachers.
A plotline might involve the magical community's reaction. Since Grindelwald was a feared dark wizard, when the news spreads that Harry is his grandson, there could be a lot of chaos. Some might fear that Harry has inherited Grindelwald's dark tendencies. However, Harry could use this situation to prove that one's upbringing and choices matter more than blood relation. He could embark on a journey to clear the name of the Potter family and show that he is different from his grandfather.
Harry might cope by retreating into his own thoughts and memories of his parents and the wizarding world. For example, he could think about the good times at Hogwarts to endure Vernon's torture.
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.