The reaction would be extremely negative. The 'Harry Potter' universe has a strong sense of family and friendship. Hermione is an important part of Ron's friend group, and Ron's dad is like a parental figure in their world. A fanfiction with this inappropriate plot would not only be against the essence of the story but also against the moral and ethical values that the series promotes. Fans would likely shun any such work.
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.