One common theme is redemption. Many fanfictions focus on Dudley changing his ways after years of being a bully. They show him realizing the error of his actions and trying to make amends.
Yes, there are quite a few. You can start by looking on popular fanfiction websites like FanFiction.net or Archive of Our Own. Some might explore Dudley's sister in an alternate universe where she has a more complex personality, perhaps befriending the wizarding world characters or having her own magical adventure in a world that her family has always shunned.
One great Dudley Dursley story is when he finally realizes how mean he has been to Harry Potter. It shows his growth as a character. For example, in the later parts of the series, Dudley starts to see Harry in a different light and even shows a bit of kindness, which is a big change from his bullying ways at the start.
One great Dudley Dursley fanfiction is 'A Different Dudley' which shows Dudley growing and changing in unexpected ways. It really delves into his character and gives him more depth than in the original series.
Since 'lemon' fanfiction is not something I can recommend due to its nature, you could start by exploring fanfiction tags related to Dudley Dursley such as 'character study' or 'alternate universe'. This way, you can find fanfictions that focus on different aspects of his character and his relationships in a more appropriate and positive way.
One way is to give Dudley a moment of self - reflection. Maybe he overhears some cruel remarks about himself from other Muggles who are as narrow - minded as his parents. This makes him realize how wrong his family's treatment of Harry has been, and he starts to change. He could then begin to learn about the wizarding world from Harry in secret, slowly becoming more open - minded and kind.
One common theme is redemption. Dudley has been a bully to Harry for a long time, so many fanfictions explore him seeking redemption for his past actions. Another theme is family. Despite their difficult relationship, they are family, and fanfictions often play with this idea, showing how they can come to terms with each other.
There might be a fanfiction where Dudley gets involved in a magical adventure accidentally. For example, he stumbles upon a magical object that was meant for Harry but now has drawn him into a dangerous yet exciting situation in the wizarding world. He has to learn to rely on his own courage and the help of some unlikely allies.
Often, there's the theme of redemption. If she's been part of the Dursley's unkind treatment of Harry in some fanfictions, she might later try to make up for it. There's also the theme of the Muggle - wizarding world divide. As a Dursley, she's on the Muggle side, but she could be curious about the wizarding world or even have some connection to it that she has to explore while dealing with her family's prejudices.
In the story where Dudley is on a diet and the magic pudding appears, it's a really good Dudley Dursley story. He is so focused on his diet but then this magical pudding comes along and completely throws him off. It shows his struggle with self - control and also how the magic in Harry's world can disrupt the normal order of things in the Dursley household. This also reveals more about Dudley's character, especially his relationship with food and his desire to fit in with his family's idea of a proper lifestyle.