One common theme is friendship. Female characters often form deep friendships with Zenitsu, helping him grow and overcome his insecurities. Another theme is adventure. They go on demon - hunting adventures together. For example, in many fanfics, they explore new regions in search of powerful demons.
Sure. One popular female Zenitsu fanfic might be 'Zenitsu's New Adventure with a Female Friend'. It often shows how Zenitsu's unique personality interacts with a female character in different scenarios, like during a demon - hunting journey. Another could be 'Zenitsu and the Female Apprentice', which delves into their relationship as mentor - mentee.
A simple Nezuko and Zenitsu fanfiction could be that they go on a journey together. Along the way, they encounter other demon slayers who are initially skeptical of Nezuko. But Zenitsu stands up for her, explaining that she's different. This shows the depth of their friendship and how Zenitsu has come to trust Nezuko completely.
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.