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.
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.
In some fanfictions, a crossover like this could lead to a romantic relationship between Naruto and Nezuko. However, this would be a very different kind of relationship as they come from such distinct backgrounds. Their relationship could be a slow - burn one, starting from mutual respect and growing into love as they face various challenges together.
One popular theme is their relationship growth. Writers often explore how Tanjiro and Nezuko's bond changes as they face different challenges in the demon - slaying world. It could be about how Nezuko's demon nature affects their connection.