The main characters are Dudley and Harry. Dudley, who has usually been portrayed as a bully in the original series, is now the hero. Harry, on the other hand, is the target of whatever danger is present in the fanfiction. There might also be other supporting characters like Dudley's parents or some of Harry's friends from Hogwarts who could play a role in the story.
Perhaps Dudley finally stands up to his father's cruel treatment of Harry. Maybe he realizes that Vernon's behavior towards Harry is wrong and uses his own physical strength or some clever ruse to get Harry out of a difficult situation Vernon has put him in.
One possible plot point could be Dudley finally realizing the true nature of the danger Harry is in and using his new - found courage to save him. Maybe he discovers some secret about Voldemort that helps him protect Harry. Another could be Dudley using his physical strength, which he was known for in the original series in a more heroic way to fend off Death Eaters.
One possible fanfic could be where Harry visits Uncle Dudley after the war. Maybe Dudley has had a change of heart and they start to build a tentative relationship. Harry could be there to show that he has moved on from their past differences and is willing to give Dudley a chance to be part of his life in a new way.
Well, a fanfiction involving Harry Potter, Dudley, and 'Cartoons Ninja' could be really interesting. Maybe it could be a story where Dudley somehow gets involved in the magical world of Harry Potter through a portal created by a 'Cartoons Ninja' character. The ninja could have some unique powers that interact with the magic in the Potterverse.
There are fanfics that take place during the later years of Harry's life, where Dudley has grown up and has a family of his own. The story could revolve around how their past still affects them and how they try to move forward. Maybe Dudley's kids end up being interested in the wizarding world that their uncle Harry is a part of, and it creates a new connection between the two families.
Another 'Harry Potter and Dudley' fanfiction might focus on an alternate universe where Dudley was also sent to Hogwarts. It would be interesting to see how Dudley coped with the magical environment, and how his relationship with Harry developed within the school walls. Would he be sorted into Slytherin? And how would that change his character?
Sure. In the world of 'Harry Potter' fanfiction, the concept of Dudley as a wizard isn't unheard of. Some fanfictions might use this idea to show Dudley's redemption arc. He could start as the bully we know from the original books but then transform into a wizard and learn about the values of the wizarding community. It can also be a way to explore the Dursley family's relationship with magic in a different light, rather than just being Muggles who shun it.
Dudley is interesting because he was a bully in the original story. Fanfiction can explore his redemption arc. He has the potential to grow and change, which makes for great storytelling.