Perhaps there is a rivalry between the aikido and kenjutsu practitioners. After the sword is batted away, the kenjutsu user becomes enraged and challenges the aikido user to a more formal duel. This duel could be the centerpiece of the fanfic, with detailed descriptions of their moves and the tension building as the fight progresses.
One possible plot development could be the discovery of a hidden martial arts technique. For example, the character who bats away the kenjutsu sword might reveal that they have been secretly studying a rare form of aikido that allows them to do such a thing. This could lead to others wanting to learn from them, creating a sub - plot of mentorship and training.
A master of both aikido and kenjutsu could be the main character. This person has reached a level where they can use the principles of aikido to neutralize a kenjutsu attack. They might be a reclusive figure, but after this incident, they are drawn back into the martial arts community to share their knowledge or to face an old enemy who is threatened by their skills.
In a fanfiction where Naruto is sealed away, there could be a focus on the emotional turmoil of his friends and family. Hinata, for example, might be extremely distraught and use her Byakugan in new ways to try and find Naruto. Sasuke could also return to the Leaf Village out of loyalty to Naruto and join the efforts to unseal him. The story could also explore how the younger generation of ninjas, inspired by Naruto's legacy, strive to become stronger to free him.
One possible plot development could be that the village is in great danger from an enemy too powerful to defeat directly. Naruto, seeing no other way, seals himself away to act as a last - line - of - defense deterrent. If the enemy tries to completely destroy the village, Naruto's sealed power could be unleashed to counter them.
Maybe Naruto's learning of Uzumaki Kenjutsu is a result of some hidden Uzumaki clan heritage coming to light. He could be guided by an old Uzumaki clan member or some ancient scrolls. As he progresses, he faces challenges from enemies who fear his newfound power. And he has to use his kenjutsu skills to protect his friends and the village.
One possible plot development is that Nagato and Naruto form an alliance to take on a new, greater threat. This threat could be an ancient evil that requires both Nagato's power and Naruto's spirit. Another development could be Nagato using his abilities to help Naruto unlock new powers or forms. For example, he might help Naruto better understand the Sage of Six Paths' power within him.
One possible plot development could be Naruto's ideals getting warped due to some traumatic event. For example, if he fails to save a dear friend, he might start to believe that the only way to achieve true peace is through extreme force. Another development could be his relationships turning sour. Maybe his friends start to fear his new, twisted self and Naruto, feeling betrayed, becomes even more deviated from his original self. Also, his view of the ninja world could change completely, seeing it as a place that needs to be remade in his own image rather than following the traditional ways.
One possible plot development could be the initial shock and adjustment for both Naruto and Mei. They would have to deal with the reactions of their respective villages. Naruto's friends in Konoha might be surprised, and Mei's allies in Kirigakure would also have their thoughts.
One possible plot development could be Naruto leaving due to a misunderstanding. For example, if he is wrongly accused of a crime within the team. Another could be that he feels he is holding the team back in some way and decides to leave on his own to become stronger alone. Maybe he has a personal goal that he believes he can't achieve while still being part of Team 7.
One possible plot development could be Arya reuniting with her family members. She might share her adventures and skills she has acquired during her time away. Another could be her getting involved in the political affairs of Winterfell, using her wits to help protect the castle and its people.
One possible plot development could be that his friends are left to pick up the pieces. They might struggle with grief and guilt, thinking they could have saved him. Another could be that Voldemort finally takes over completely without Harry to oppose him, leading to a dark reign over the wizarding world.