One possible plot could be that a powerful villain tries to create a new Buu - like being using Naruto's chakra. But instead of creating a new Buu, Naruto is reincarnated as a hybrid of himself and Buu. This would lead to a lot of internal struggle for Naruto. He would have to deal with Buu's instincts while still trying to protect his village and friends. His friends might be initially scared of him, but over time they come to understand and accept this new Naruto.
It could be someone who wants to use Naruto's new Buu - like powers for their own evil purposes. Maybe an old enemy who realizes that Naruto in this new form is a huge threat to their plans. For instance, Orochimaru might see this as an opportunity to gain even more power and try to control Naruto - as - Buu.
Uub is often characterized as a pure - hearted character in Dragon Ball Super fanfic. His origin as the reincarnation of Majin Buu gives him an interesting backstory. He's usually shown as having a strong sense of justice, much like the other heroes in the Dragon Ball series.
There could be a uub and pan fanfiction where they meet for the first time in a strange place. Maybe it's a dimension that's a crossroads between their respective realms. They start off as strangers but gradually become friends as they learn about each other's powers and cultures. This type of story can focus on their character development and the building of their friendship.
Another place to search is FanFiction.net. It's been around for a long time and has a wide range of fan - created stories. Just use the search function and type in 'uub and pan' to see what comes up. However, the quality can vary, so you might need to do some sifting through the results.
The Naruto - Kid Buu fusion in fanfiction could be depicted as a being with a unique fashion sense. Since Naruto often wears orange and Kid Buu is mostly pink, the fused form might have a combination of those colors in its clothing or body markings. In terms of abilities, it could be shown as having the speed of Naruto when he uses his chakra to enhance his movement, combined with Kid Buu's ability to transform parts of his body into weapons. For example, the fused form could create Rasengan - like attacks from its hands which are also able to stretch like Kid Buu's arms.
Super Buu is definitely comical. His childish tantrums, odd behavior, and the way he reacts to situations often provide comic relief. However, it also depends on how you interpret his character and the context of the story.