Another interesting approach is to focus on the origin of the Shiki Fujin in fanfiction. Maybe create a backstory where it was created by an ancient clan related to Naruto's Uzumaki heritage. This could open up new plotlines and mysteries.
One possible plot twist could be that Naruto initially struggles a great deal with Shiki Fujin. Maybe he accidentally activates it in a wrong way and it starts to drain his chakra in an uncontrollable manner. This could lead to a situation where his friends have to step in to save him, and in the process, he discovers a new way to control the jutsu.
A possible plot could be that the devil fruit that Shiki has is a Logia - type devil fruit which allows him to turn into a certain substance. In the Naruto world, this confounds Naruto and his friends as they've never faced such an ability before. Shiki uses this power to infiltrate villages undetected. Naruto starts to study the nature of the devil fruit's power with the help of some of the more knowledgeable characters in the series, like Jiraiya (if he's still alive in this fanfiction). They eventually find a weakness in Shiki's power, which lies in a certain type of chakra that can disrupt the devil fruit's transformation process.
Incorporate unexpected twists. For instance, instead of the Shiki Fujin being a last - resort move, make it a move that has some unforeseen side effects that Naruto has to deal with throughout the story. This will keep the readers on their toes.
There could be a fanfiction where Naruto and Fujin go on a journey together to discover a lost ninja technique. Along the way, they face various challenges and their relationship grows as they learn to trust and rely on each other.
Naruto's character could become more somber. Learning Shiki Fujin, a very dangerous and potentially self - sacrificing jutsu, might make him more aware of the weight of his responsibilities. He might start to think more about the consequences of his actions and be less impulsive.
A really interesting plot could be a sort of 'what - if' scenario. For example, what if Shiki and Rima were to switch their roles or abilities? How would they adapt to this new situation? It could lead to some really creative storytelling, with them having to deal with the challenges and advantages that come with this role - reversal while still maintaining their bond.
One idea could be a story where Akira and Shiki are forced to team up against a new threat in the Togainu no Chi world. Maybe a powerful and mysterious figure emerges, and they have to put aside their differences to protect their territory.
One interesting plot could be the exploration of new jutsu by the Kage Fujin. Maybe they develop a unique way to combine different elemental chakra in a never - before - seen manner.
In some fanfictions, Naruto Shiki might possess the power to manipulate shadow elements in extremely unique ways. For example, he could use his shadow to not only mimic the movements of others but also to create shadow constructs that can be used in combat, like shadow swords or shields.
One interesting Nano x Shiki fanfiction could be about their adventures in a magical realm. They might encounter strange creatures and have to use their unique skills to survive. For example, Nano's small size could be an advantage in sneaking around, while Shiki's strength could help in battles.