One interesting plot idea could be Dudley traveling back in time to when his parents first got married. He could see how his family's attitudes towards magic and the Potters were formed. Another could be him traveling forward in time to see the impact of Harry Potter's actions on the wizarding world and how it indirectly affects the Muggle world he lives in. Maybe he travels to a time when a major wizard - Muggle conflict was about to start and he has to use his new - found knowledge of both worlds to prevent it.
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.
It could be Dumbledore in his younger days. Dudley would be really shocked to meet the great wizard when he was still young and full of energy. Dumbledore's wisdom and power would be a complete contrast to what Dudley is used to in his Muggle life.
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.
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 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.