Many of the younger shinobi who looked up to Naruto would be disheartened. His decision to quit could lead to a change in the shinobi culture. Some might question their own motives for being shinobi. There could be a shift in power dynamics as well. Other villages might see Konoha as being weaker without Naruto and might try to take advantage. On the other hand, Naruto's departure could also open the door for new heroes to emerge within the village, but it would be a difficult transition.
On a broader scale, the balance of power in the ninja world would shift. Other villages might see Konoha as being weakened. This could lead to potential threats or changes in alliances. Moreover, Naruto's decision to quit could inspire some other shinobi to question their own roles and values, leading to a wave of self - reflection and possible changes in the shinobi way of life across different villages.
The story would lose a strong fighter. Her friends and comrades would have to adjust without her. They might struggle more in battles and lack her unique skills and strategies.
Maybe he realizes that the shinobi world is too corrupt. There could be political intrigue and power struggles that go against his ideals. He has always fought for peace and unity, but if he sees that the system is beyond repair, he might choose to quit. Also, he could have a family situation that requires him to step away from the shinobi life. For instance, if he has a child with special needs that demands his full attention.
Well, the shinobi rules in Naruto fanfic can be quite diverse. There's the rule of secrecy regarding jutsu. Shinobi are not supposed to freely share their unique and powerful jutsu with outsiders or enemies. Then there's the code of honor in combat. They should fight fairly, not use underhanded tactics like attacking an unarmed opponent. And of course, they are required to respect the hierarchy within the village's shinobi system.
One reason could be a sense of not being understood. Maybe Naruto feels like his teammates, Sasuke and Sakura, don't truly get him. Sasuke is often preoccupied with his own revenge, and Sakura has her eyes mainly on Sasuke. Naruto might feel left out and that his goals are not aligned with the team's, so he decides to quit.
Perhaps it's a shinobi who has retired after a long and eventful career. He could be someone who has seen the darker side of the shinobi world and wants to make sure Naruto doesn't go down the wrong path. He might have been a wandering shinobi who finally settles down with Naruto. For example, he could be a shinobi from a different village who has a unique set of skills to teach Naruto.
There would be a group of people who understand his decision. They might have seen how much Naruto has sacrificed and how tired he must be. They would support him in his choice to step down and hope he can finally have some peace in his life. For instance, his close friends like Shikamaru might be among those who support his decision.
The storyline would be completely different. Naruto's character development would take a new turn. He would no longer have the instant boost of power from the Nine - Tails. This might make him more vulnerable, but also force him to rely more on his own skills and training. His interactions with the other Jinchuuriki would also change. They might view him as a traitor to their kind or as a pioneer for a new way of living without the tailed beasts.
One possible plot twist could be that Naruto discovers a new power source that makes being a Jinchuuriki seem obsolete. Maybe he finds an ancient artifact or a new form of chakra that is more powerful and doesn't have the drawbacks of being a Jinchuuriki. Another could be that he has a moral dilemma about the nature of the Jinchuuriki system and decides to quit in protest. For example, if he realizes that the Jinchuuriki are being used as tools more than protectors.