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 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.
Well, a frequent theme is Harry's resilience. In these fanfictions, he has to endure the Dursleys' cruelty, yet he doesn't break. For example, he might secretly practice magic to give himself some hope in the face of their abuse. Another theme is the exploration of the Dursleys' motives. Some fanfictions dig deeper into why they are so cruel to Harry. Is it just pure dislike or is there something more sinister? And finally, the theme of justice often appears. Fans might write about how Harry eventually gets revenge or at least stands up to the Dursleys in a powerful way.
Well, a major theme is the contrast between the magical world Harry belongs to and the Muggle world of the Dursleys. In these fanfictions, the abuse by the Dursleys emphasizes how different Harry is from them. It also often explores the concept of justice. Fans might write about how Harry eventually gets the justice he deserves, either by standing up to the Dursleys or having the wizarding world intervene. And there's the theme of friendship. Harry's experiences with the Dursleys make him value true friendship even more when he finally gets to interact with people like Ron and Hermione.
In some fanfictions, the Dursleys might be portrayed as having a change of heart. For example, they could start to secretly admire Harry's magical abilities instead of resenting them. This can add an interesting twist to the original story.
Dumbledore might play a role. He could be the one who encourages Harry to stand up to the Dursleys. He might have some hidden knowledge about the relationship between the wizarding world and the Dursleys that could help Harry. Also, Hagrid could be involved. He could provide some muscle if things get physical. He might also offer some emotional support to Harry, as he's always been like a big, friendly giant to him.
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.
One common theme is Harry's struggle with self - worth. After being abused by the Dursleys, he often has a very low opinion of himself in these fanfics. Another theme could be his search for a real family or a place where he belongs. Since the Dursleys treat him so horribly, he might be constantly on a journey to find people who will love and accept him unconditionally.
One prank could be Harry using magic to make all of Dudley's clothes two sizes too small overnight. Another might be making the Dursleys' living room furniture float whenever they try to sit down. And he could also use magic to make the food on their table turn into slugs when they're about to eat.