You can try some fanfiction websites like FanFiction.net or Archive of Our Own. They usually have a large collection of fan - made stories including those related to mugiwara.
As mentioned before, using well - known and legal fanfiction platforms like FanFiction.net and Archive of Our Own is a legal way. These platforms have rules and regulations in place to ensure that the content is within the bounds of legality, mainly in terms of respecting copyrights and not including inappropriate or illegal content.
Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.
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.