One great Kamisama Kiss Mature fanfiction could be the one that focuses on the deeper emotional bond between Nanami and Tomoe as they face more complex challenges in their relationship. It might explore how they grow and change in a more mature way compared to the original story.
You can start by looking on popular fanfiction websites such as Archive of Our Own (AO3). It has a large collection of fanfictions, and you can use the search function to specifically look for Kamisama Kiss Mature fanfiction.
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.
There might be some fanfiction on FanFiction.net as well. However, the quality can vary greatly. Look for stories with high ratings and reviews. Some authors are really good at exploring the concept of Nanami being pregnant in the context of the Kamisama Kiss universe, adding new plotlines and character developments.
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 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.
One common theme is the exploration of Nanami's relationship with Tomoe. When she is pregnant, it often adds a new layer of complexity to their relationship. For example, Tomoe might become more protective.
A crossover with 'Fruits Basket' could also be cool. Nanami could end up in the Sohma family's world and befriend Tohru. Tomoe might be a bit jealous at first, but then he gets to know the Sohmas and they all work together. Maybe there's a curse involved that Nanami and Tomoe try to help break with the Sohmas' unique situation.
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.