One great fanfiction idea could be that Nanami and Tomoe go on a journey to the spirit world to solve a mystery that threatens the human world. Along the way, they face various challenges that test their relationship and their individual powers.
One reason could be that its distribution was limited. Another possibility is that there aren't many reprints, making it scarce. Also, it might be more popular in certain regions and less accessible elsewhere.
One common theme is the exploration of different worlds. Since Kagome comes from one world and the Kamisama Kiss characters are from another, fanfics often focus on her experiencing the new world of Kamisama Kiss.
The 'kamisama kiss light novel' tells the story of Nanami, an ordinary girl who suddenly becomes a god. She has to deal with the duties of a god while also navigating her feelings for Tomoe, a fox spirit. The story is filled with magic, humor, and heartwarming moments as she tries to fit into this new world.
There might be a Kamisama Kiss fanfiction where the story is told from the perspective of a minor character. For example, a sidekick spirit could tell their version of the events that unfold in the main plot. This could give a fresh and unique view on the relationships and the magical world of Kamisama Kiss.