Nezuko remains alive in the Demon Slayer manga. The author chose to keep her alive as she is an important character with a vital part to play in the overall narrative.
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.
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.
A Nezuko and Zenitsu fanfiction might explore the concept of Zenitsu teaching Nezuko about the human world. Since Nezuko has been a demon for a while, she may have forgotten some aspects of human life. Zenitsu could show her simple things like how to enjoy a flower's beauty or the taste of human food (in a non - harmful way for her). This could create a really sweet and heartwarming story as they bond over these new experiences.
One common theme is protection. Since Nezuko is a demon and Zenitsu is a demon slayer, there are often stories about Zenitsu vowing to protect Nezuko from other demon slayers or more dangerous demons. Another theme is friendship. They develop a unique bond in the original story, and fanfics often explore that further, like how they support each other through tough times. And of course, there can be a bit of a romantic undertone in some fanfics as well.
Definitely not. The story doesn't depict Zenitsu and Nezuko as a couple. Their paths in the manga are more about their individual quests and battles against evil.
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.