There was this girl who always had her headphones on. One day, she sat by the window. She took a deep breath in, and the music in her headphones seemed to amplify. It was a beautiful melody. As she breathed out, she closed her eyes. In her mind, she was in a meadow, the music creating the scene. With each inhale, she felt more alive, and with each exhale, she drifted further into the story the music was telling.
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.
Definitely not. In the manga, there's no such plotline of Zenitsu and Nezuko getting married. Their relationship is more focused on being comrades in the fight against demons rather than a romantic one leading to marriage.