One popular 'zenitsu x nezuko fanfic' is 'The Unlikely Bond'. It beautifully portrays the relationship between Zenitsu and Nezuko. The author does a great job of showing Zenitsu's protective nature towards Nezuko and how Nezuko's presence calms Zenitsu in his often-fearful state.
Well, a lot of it has to do with the potential for character development. In the original story, their interactions are limited, but in fanfics, authors can explore how they would grow closer and influence each other. For example, Zenitsu could become more confident because of Nezuko, and Nezuko could learn more about the human side of relationships through Zenitsu. It's also about the fans' love for the two characters and their desire to see them in different scenarios together.
One common theme is self - discovery. Since Zenitsu has a complex personality, many fanfics focus on him finding out more about himself, his powers, and his place in the world. Another theme is friendship. He often forms strong bonds with other characters in fanfics, like Tanjiro and Inosuke.