In this fanfiction scenario, it could be that the zanpakuto has a mind of its own. It takes control of Naruto and forces him to attack Kakashi. The zanpakuto might be from a different dimension and is trying to create chaos in the Naruto world. Kakashi would have to rely on his ninja skills and his knowledge of various jutsu to counter Naruto's attacks and find a way to free Naruto from the zanpakuto's control.
One possible plot could be that Kakashi is under a genjutsu. In this fanfiction, an enemy ninja might have cast a powerful genjutsu on Kakashi, making him see Naruto as a threat to the village. So, Kakashi attacks Naruto, but Naruto, being the understanding person he is, tries to break the genjutsu instead of fighting back right away.
One idea could be that Naruto is under a powerful genjutsu that makes him see Konoha as an enemy. So, he attacks without realizing it's his home. Another could be that an evil entity has possessed Naruto, forcing him to turn against Konoha. It might also be that Naruto has been misled by false information about Konoha's dark secrets and decides to take action against it.
Sasuke would most likely side with Naruto. Despite their past differences, they have a deep bond. Sasuke would not let Kakashi harm Naruto without understanding the situation fully. He would step in to protect Naruto and demand an explanation from Kakashi.
Kakashi could use his Earth - Style: Earth Wall jutsu to create a barrier between him and Naruto. This would give him some time to analyze Naruto's new weapon and his changed behavior. He might also use his Sharingan to predict Naruto's moves and dodge the zanpakuto strikes more effectively.
One interesting plot could be the discovery of an ancient prophecy that states a person with a specific Zanpakuto bloodline will be the key to saving the Naruto world from an impending doom. This person is initially unaware of their bloodline and must go on a journey of self - discovery. As they do, they learn about the power of their Zanpakuto and how it relates to their bloodline. They face numerous challenges, including battles with other powerful ninjas who want to use the prophecy for their own gain.
A possible plot is that Naruto discovers a hidden power within Kakashi that even Kakashi was unaware of. This discovery leads them on a journey to explore the origin of this power. Maybe it's related to an ancient clan. As they dig deeper, their relationship grows stronger and they face many challenges together from enemies who also want this power.
One idea could be a story where young Kakashi has to protect a young Naruto from a group of rogue ninjas. It could show Kakashi's early skills and Naruto's innocent yet determined nature.
A really interesting plot for 'kakashi and naruto family fanfiction' could be that Kakashi accidentally stumbles upon a secret power within Naruto's family that has been dormant for generations. He then helps them unlock and control this power. Along the way, they form a stronger bond as a family. And as Kakashi is involved, he also rediscovers some of his own values and beliefs through this process.
One plot idea could be that the female Naruto has a special training session with Kakashi. During this, she discovers a new jutsu that Kakashi has been secretly working on. He decides to teach it to her because he sees her potential. This new jutsu then becomes crucial in a future battle against a powerful enemy.
One idea could be that little Naruto gets lost in the forest and Kakashi has to find him. Along the way, Kakashi discovers Naruto's secret hiding place filled with his dreams and wishes written on little scrolls.