In a Tomoe x Nanami fanfiction, it would be interesting to write about a situation where they have to pretend to be a married couple for some spirit - world related reason. They would have to deal with all the implications of that, like sharing living quarters and acting affectionate in public. This would force them to confront their true feelings for each other.
A possible fanfiction plot could be Nanami accidentally loses her powers and Tomoe has to protect her while also trying to find a way to restore her powers. In the process, they discover new things about each other and their feelings grow stronger. It could be a story full of action, a bit of angst, and a lot of tender moments.
I really like the fanfic 'Tomoe and Nanami: Unspoken Promises'. It focuses on the more subtle moments between them. There are many sweet scenes where Tomoe shows his protectiveness over Nanami in his own unique way. It's a relatively short but very engaging read.
A possible plot could be that Nanami gets into trouble with a powerful spirit and Tomoe has to make a great sacrifice to save her. This sacrifice could be something like giving up a part of his power or his memories. It would show the depth of his love for her.
In these fanfics, we often see the deepening of their emotional bond. Tomoe's initial coldness towards Nanami gradually melts, and seeing how they overcome various obstacles together in fanfics is really engaging. Also, the cultural elements from the anime, like the Japanese mythology and traditions, add more charm to the stories.
A great element to include is the magic that exists in the Kamisama Hajimemashita universe. Nanami learning new spells or Tomoe using his powerful yokai magic in unexpected ways can make the story exciting. You could also have a sub - plot where a rival for Nanami's affections appears, either a human or a yokai. This would create jealousy and tension between Nanami and Tomoe, leading to some passionate moments and a stronger development of their relationship.
In the ending story, Nanami's growth as a kami is a significant event. She has proven herself capable. Also, the relationships between all the characters, not just Nanami and Tomoe, are solidified, bringing harmony to the story's world.
One popular trope is the 'reincarnation' trope. In these fanfictions, characters might be reincarnated in different time periods or under different circumstances, which often leads to new and interesting storylines. For example, the main character could be reincarnated as a different deity or a human with some sort of connection to the original story.
You can try websites like Fanfiction.net or Archive of Our Own (AO3). These platforms usually have a large collection of fanfictions for various series, including Kamisama Hajimemashita.