A great 'Harry Potter Dursleys fanfic' might explore the Dursleys' backstory more deeply. For example, it could show how Vernon's hatred for magic and strange things developed over time. Or it could focus on Petunia's envy of Lily's magical abilities and how that affected her relationship with Harry. Some fanfics also imagine a scenario where the Dursleys are magically transported to Hogwarts and have to adapt to the magical environment. They would be completely out of their element, and it would be interesting to see how they interact with the students and teachers.
In some of these fanfics, one interesting plot could be Harry finding solace in an unexpected friendship with a local magical creature that the Dursleys were unaware of. This gives him a sense of purpose and connection outside of the neglect he faces at home.
In some 'Harry Potter and the Dursleys' fanfics, there's a plot twist where the Dursleys are actually protecting Harry from a greater threat that the wizarding world doesn't even know about. They've been cruel to keep him close and safe. Also, there could be a twist where Vernon Dursley has a connection to the wizarding world through his family's past that he's been trying to hide all along, which changes how we view his hatred for magic.
One interesting Dursleys fanfiction could be about Dudley's life after learning the truth about the wizarding world. Maybe he starts to have a change of heart and tries to make amends for how he treated Harry over the years.
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.
Perhaps in the fanfic, a time - traveling wizard goes back to save Harry from the Dursleys. This would lead to a lot of changes in the timeline. Harry might be introduced to magic much earlier and form different relationships. He could also have a different understanding of his parents' history and his own role in the wizarding world as a result of being removed from the Dursleys' environment.
One possible plotline could be how the Dursleys face magical mishaps in their daily lives that drive them crazy. For example, maybe Dudley accidentally gets turned into a pig by a stray spell and the Dursleys have to deal with that chaos. Another could be that they are constantly haunted by strange magical creatures that are attracted to Harry's presence in their house.
Definitely Dudley. In fanfic, he can be developed in so many different ways. He could become a wizard himself, or he could befriend Harry in a more meaningful way than in the original books. Another interesting character is Petunia. There could be more exploration of her past and her relationship with Lily, which can make her character more complex.
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.
One interesting fanfiction could be about the Dursleys having a change of heart before Harry goes to Hogwarts. Maybe they start to see the magic in him in a different way and struggle with their newfound feelings of protectiveness. It could show a different side of the usually cruel Dursleys.
One popular fanfiction might be about the Dursleys' secret past that somehow ties them to the wizarding world in a more complex way. For example, perhaps there was a wizard in their family line long ago that they've been trying to hide. Another could be a story where the Dursleys have a change of heart much earlier than in the original series and start to embrace Harry's magical nature, leading to all sorts of comical and heartwarming situations as they try to understand the wizarding world. And there could be a fanfiction where the Dursleys are forced to face the consequences of their mistreatment of Harry in a more magical and extreme way, like being haunted by magical apparitions that represent Harry's pain.