Cassidy Sharp could be a complex character in the 'Cassidy Sharp Survives' fanfic. They might be someone with a mysterious past, which could be gradually revealed as the story progresses. They could be a loner type who has to learn to trust others in order to survive, or perhaps a natural leader who takes charge in difficult situations.
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.