Well, the Dursleys could manipulate Harry's view of the wizarding world. They could tell him lies about how all wizards are evil and that he should stay away from them. They could also destroy any letters or signs from the wizarding world that come to Harry. By doing this, they cut him off from the only positive community he could belong to. As a result, Harry, feeling alone and betrayed, might start to develop a hatred towards both the wizarding world and the Dursleys, and this could lead him down a dark path.
The Dursleys could start by being even more cruel and abusive. They could constantly belittle Harry, tell him he's worthless and that he doesn't deserve any kindness. This could chip away at his self - esteem and make him more vulnerable to dark thoughts.
They could also physically and emotionally isolate him even more than they do in the original story. By cutting him off from any positive influence, like his friends at Hogwarts, they make him more vulnerable. Then, they could introduce him to some seemingly 'cool' or 'powerful' dark magic artifacts or spells, which might initially attract Harry's curiosity and eventually lead him down a darker path. For instance, they might find a way to get their hands on a dark magic book and leave it for Harry to discover.
One key theme could be the corruption of innocence. The Dursleys, who are typically neglectful and mean in the original, might take their actions to an extreme in a dark fanfiction, completely warping Harry's view of the world. Another theme could be power dynamics. They might use their position as Harry's guardians to gain some sort of dark power or influence. Also, isolation could be a theme, as they further isolate Harry from any positive influences, leading him down a dark path.
The Dursleys could also cripple Harry emotionally in fanfiction. Their constant belittling, isolating him from others, and treating him like a burden could break his spirit. This emotional crippling might make it difficult for Harry to trust people, form relationships, or even fully embrace his wizarding identity. He may become withdrawn and lack the confidence needed to face the challenges in the wizarding world.
Perhaps Harry uses his influence among his friends. Hermione could help him find some loophole in the Muggle - wizarding laws regarding property. Ron might provide some comical support, like trying to use some half - baked spell to make the Dursleys uncomfortable enough to leave. In the end, Harry evicts them and takes over the house, turning it into a place that's more welcoming for himself and his wizarding friends when they visit the Muggle world.
Perhaps Harry sues the Dursleys for false imprisonment. He realizes that all those years of being confined to his room or the cupboard were illegal. He enlists the help of Hermione, who is well - read in both wizarding and Muggle laws. They present a strong case against the Dursleys. As a result, the Dursleys face public humiliation in their neighborhood, and Harry finally gets the justice he deserves, while also starting to heal from the trauma of his childhood.
One possible plot could be that the Dursleys, out of their long - held hatred and envy, finally find a way to use dark magic against Harry. Maybe they make a deal with some dark wizards who provide them with a powerful curse. They manage to cast it on Harry when he's at his weakest moment, perhaps during one of his vacations at Privet Drive.
In this fanfiction, perhaps the Dursleys' constant abuse and neglect cause Harry to have some sort of mental or physical disability. It might show how he has to overcome not only the challenges of the wizarding world but also the trauma inflicted by his Muggle relatives. For instance, he could have problems with his legs from being locked in a cupboard for long periods, which would be a physical crippling, and then have to deal with the self - doubt and fear that comes with it.
One way the Dursleys can be good in fanfiction is if they are forced to face the consequences of their actions early on. For instance, they might receive a warning from the wizarding world that if they don't treat Harry well, there will be serious repercussions. So they start to be nicer to him. They could provide him with a proper room, better food, and allow him to have friends over.
In some Harry Potter fanfictions, the Dursleys are depicted as nice. This could be a really interesting take. It might show how they could have been different if they hadn't been so influenced by their dislike for magic. Maybe they could have supported Harry more, or at least been less cruel. For example, a fanfiction could explore a scenario where Vernon Dursleys actually encourages Harry's academic achievements at school, instead of always putting him down.
A lot of fanfictions play around with the Dursleys' relationship with Harry. They could be depicted as having a sort of begrudging respect for Harry. For instance, when Harry starts to show real courage and power, the Dursleys might start to see him in a new light. This is different from the original story where they mostly just try to suppress Harry. Also, some fanfictions might give the Dursleys magical abilities of their own, which completely changes their dynamic with Harry and the wizarding world.