It could be Dudley. He could have a character arc where he realizes the error of his ways in bullying Harry and decides to embrace the magical world. Maybe he discovers he has a unique magical talent that was hidden all along.
Obviously, Vernon Dursley could be a main character. We could explore his thoughts and feelings more deeply, especially if the fanfiction takes a different turn on his personality, like making him more sympathetic.
Another plot could be that the Dursley family gets involved in a magical adventure that has nothing to do with Harry. For example, they accidentally stumble upon a magical artifact that transports them to a different magical realm where they have to learn magic quickly to survive and find their way back home.
Another plot could be a time - travel scenario. A character goes back in time and changes the way the Dursleys treated Harry. As a result, they become a more loving and accepting family. This alteration in their relationship could have a huge impact on the entire wizarding world as Harry grows up with a more positive influence.
It could be someone completely new. A character that the author creates to disrupt the status quo in the Twilight universe. This new character might have unique powers or abilities that change the course of the story.
In many 'Harry Potter Dursley fanfiction' stories, the Dursleys are used as a contrast to the wizarding world. Their normal, mundane lives are juxtaposed with the magic and wonder of Harry's world. This can create interesting storylines where they are either forced to accept the magic or try to fight against it in their own stubborn ways.
Obviously, Daisy Dursley herself is a main character. She is often the center of the story, with her actions and experiences driving the plot forward.
Shikamaru is also a likely candidate. His high intelligence could be the key factor in this 'different destiny' story. He might use his strategic mind to outwit enemies that were previously thought to be unbeatable. For instance, he could come up with a plan to defeat Madara without relying on brute force, which would make him the central figure in the fanfic.
A little girl with extraordinary powers like the ability to control the weather. She could use her powers to save her village from a drought.
It could be Percy himself. Since he has such a rich backstory and amazing abilities as a demigod, he would likely be the center of attention when meeting Paul's family.
It could be a middle - aged woman who discovers a long - hidden family secret that shatters her perception of her family. For example, she might find out that her father was not her biological father.