The Kamisama Kiss manga is fully complete. It has covered everything the author wanted to convey, giving a comprehensive and fulfilling story. There are no loose ends or unresolved issues, making it a well-rounded manga series.
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.
There are several mangas that share similarities with Kamisama Kiss. For example, 'Fruits Basket' has a touch of fantasy and romance like Kamisama Kiss.
One possible scenario in such a fanfiction could be Nanami and Tomoe going on a journey to the spirit world together. They might face various challenges there, like dealing with mischievous spirits. Tomoe, with his powerful yokai abilities, would protect Nanami throughout the adventure. And Nanami, with her kind heart, could end up making friends with some of the spirits they encounter.