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.
One plot idea could be Jirou and Nanami getting lost in a magical forest together. They'd have to rely on each other to find their way out, facing various magical creatures along the way. This would test their relationship and maybe lead to some romantic moments as they huddle together for safety at night.
A plot idea could be that Nanami accidentally loses her memories and Tomoe has to help her regain them. He would take her to all the places that are important to their relationship, like the shrine where they first met. Along the way, Tomoe would be patient and kind, and as Nanami slowly starts to remember, their bond would become even stronger.
One idea could be that Nanami's sister discovers a hidden power within herself that is different from Nanami's. Maybe she can communicate with a special type of spirit that no one else can. This could lead to her getting involved in some adventures that are parallel to Nanami's but also unique in their own way.
One idea could be that the reader is a new shrine maiden and Mizuki has to teach them the ropes. They could start off not getting along because the reader is stubborn and wants to do things their own way, but over time they develop a deep bond as they face various challenges at the shrine together.
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.
A plot could be where Nanami and Tomoe accidentally swap powers for a day. Nanami has to deal with Tomoe's fox powers and all the mischief that comes with it, while Tomoe has to handle being a land god and dealing with human requests. It would be a comical yet also a character - building situation.
In a crossover with 'Naruto', Naruto himself would be a great addition. His positive and determined attitude could contrast with Tomoe's more reserved nature. And his powerful ninja skills could come in handy when facing some of the demons in the Kamisama Kiss world. Sakura could also be there, and she might bond with Nanami over their strong wills.
One idea could be a story where they go on an adventure together to save their world from a dark magic. They face many challenges and in the process, their relationship deepens.
One idea could be that characters from 'kamisama no inai nichiyoubi' cross over into a different magical world. They might have to adapt to new rules and powers while trying to find a way back home. For example, the main character could meet new allies who help them understand the strange new environment.
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.