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.
Well, it could be that a group of muggle - born wizards, who have a special connection to Harry's mother, find out about his situation at the Dursleys and rescue him. They then take him on a journey to discover his true heritage in a more positive and inclusive way. This would give Harry a more diverse view of the wizard - muggle relationship from the start.
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.
One possible plotline could be Harry finding a way to secretly practice magic while imprisoned. For example, he might discover a hidden corner in the attic where he can perform small spells without the Dursleys noticing.
In certain fanfictions of this type, Harry could be found by a powerful but reclusive wizard. This wizard decides to mentor Harry in the ways of ancient and lost magics. Harry then uses this knowledge to not only become a very powerful wizard but also to change the wizarding world's view on things like blood purity. He may also go on a journey to find out why the Dursleys abandoned him and in the process, uncover some dark secrets about his family that had been long buried.
One possible plotline could be Harry discovering a hidden magical ability that helps him escape. For example, he might find out he can teleport short distances, which he uses to get away from the Dursleys' house when they are being particularly cruel. Another could be that he gets help from an unexpected magical creature that has been secretly observing him all along. Maybe a house - elf that takes pity on him and shows him a secret passage out of the house.
One possible plotline could be Harry finding a way to break free from the chains and use his magic to confront the Dursleys about their mistreatment. Another could be the Dursleys realizing the error of their ways after chaining Harry up and having a change of heart. Or perhaps there are magical forces at work that lead to a series of adventures while Harry is still chained, like dealing with strange creatures that are attracted to the situation.
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.
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 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.
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.
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.