They could be introduced as antagonists. Say, a group of rogue ninjas who want to steal the sword for their own evil purposes. They could be led by a powerful and menacing character who has a personal vendetta against the protagonist because of the sword's power. When they first appear, they could cause chaos and destruction, setting up the conflict.
A new Akatsuki member, a young and powerful ninja who has a personal vendetta against Naruto. Their clashes could add a lot of drama and action to the fanfic.
There could be a group of orphans who have banded together and developed their own unique style of ninjutsu out of necessity. These orphans could add a new dimension to the story as they struggle to survive in a world full of powerful ninjas while also trying to find their place in it.
There could be a new neighbor in Jenna's apartment building. A quirky artist who inspires Jenna in new ways, either in her personal life or her career at the magazine.
In Naruto fanfiction, new characters can be introduced through a mystery or a prophecy. For example, there could be a prophecy about a powerful ninja who will come to change the fate of the ninja world. This then leads to the introduction of a new character who is believed to be that person from the prophecy. Another way is through a political situation. A new character could be introduced as a diplomat or an envoy from another village, bringing new political dynamics and storylines into the fanfiction.
A new character could be an old hermit who lives in a hidden cave in the mountains. This hermit has mastered a unique form of chakra control that allows him to manipulate the elements in ways not seen before. He takes an interest in Team 7 and decides to train them. He teaches Naruto how to combine his wind chakra with the earth beneath him to create powerful tornado - like attacks. For Sasuke, he shows how to infuse his fire chakra with lightning to make it more potent. And for Sakura, he helps her develop a new form of medical ninjutsu that can heal internal injuries more quickly.
The bone sword in fanfiction sometimes has a sentient nature. It can communicate with the wielder, guiding them in battles or revealing secrets about the ninja world. This adds an interesting dynamic as the character has to not only master the physical aspects of the sword but also understand its mind.
If the anime wasn't enough, then hurry up and watch the novel version of " Sword Comes "! The original novel was equally wonderful!
There could be a new character who is a descendant of an ancient clan that once had the Rinnegan. This character might show up during the Chunin Exams to either help or hinder the current Rinnegan user. They might have some knowledge about the Rinnegan that others don't. Also, an anti - Rinnegan organization's agent could be introduced. They would be there to try and eliminate the Rinnegan user during the exams, thinking they are a threat to the balance of power.
One possible main plot could be a character in the Naruto universe who discovers a special sword that has a spiritual connection to them, stating 'I am the bone of my sword'. This could lead to a journey of self - discovery as they explore the powers and history of the sword. Maybe they have to face off against an enemy who also covets the sword's power.
Sasuke is an obvious choice. He's already associated with swords like the Kusanagi. A fanfic could explore his further development with the sword, perhaps unlocking new abilities or techniques.