Perhaps Naruto in the Anbu has a personal mission that goes against the village's orders. For example, he might befriend someone from an enemy village who is actually innocent but is being targeted by Konoha. When the village refuses to listen to his pleas, he decides to leave. He'll use his Anbu abilities to protect his new friend and find a new place where they can both be safe, away from the political drama of the ninja villages.
It could be a rogue Anbu member. This person might be jealous of Naruto's power and popularity within the Anbu. So, they spread false rumors about Naruto which lead to his decision to leave the village. This antagonist would then try to further harm Naruto while he's outside the village, using their own Anbu skills against him.
Perhaps the ANBU has a personal vendetta against someone in Konoha who is too powerful to take on while remaining in the village. Leaving Konoha gives them the freedom to train and gather allies to come back and face their enemy. They might leave secretly, under the guise of a mission, and start building their strength in the outside world, all while Konoha is left in the dark about their true intentions.
In this fanfiction, Naruto might be neglected not just by the villagers but also by some of his so - called friends. This heartbreak makes him leave. He then travels to different lands. Along the way, he meets an old hermit who teaches him ancient ninja techniques that are long forgotten in his village. With these new skills, he contemplates whether to return to the village and show them what they missed out on or to start a new life away from the place that has caused him so much pain.
In one possibility, Naruto might leave the Leaf due to a misunderstanding. For example, if the villagers continue to show distrust towards him despite his efforts. He could set out on a journey to find a place where he truly belongs. Maybe he meets some new allies on his way, people who understand him without any prejudice.
In some fanfictions, Naruto might leave the Anbu due to internal conflicts within the organization. There could be corruption or power struggles. Naruto, being the idealistic character he is, would not want to be part of such a messed - up system. He might have discovered some dark secrets about the Anbu that made him decide it was not the place for him.
He could feel a sense of guilt. Sasuke might think that his actions or their relationship issues pushed Naruto to leave. He may then set out to find Naruto, not only to bring him back but also to resolve their long - standing issues. For example, if there were unspoken words or unresolved fights between them, Sasuke would see this as an opportunity to set things right.
One possible plot could be that Naruto is discovered to have an extraordinary natural talent that is recognized by a powerful figure, like the Third Hokage. This figure decides to fast - track Naruto's training and place him in the ANBU. In the ANBU, Naruto could face discrimination from the other members due to his age and being the Jinchuriki. But he overcomes this with his determination and starts to show his unique abilities, like his unrivaled stamina. He might be sent on missions where his innocent perspective actually helps in solving problems in unexpected ways.
Maybe in the fanfic, Team 7 in ANBU is tasked with protecting a powerful and important shinobi from another village who has come to Konoha for peace negotiations. Naruto tries to befriend the visiting shinobi, Sasuke is constantly on guard against any possible threats, and Sakura is ready to heal any injuries in case of an attack. There could be internal conflicts within Team 7 as they deal with different approaches to protection.
Well, one possible plot could be Naruto starting from scratch. He might first gather like - minded ninjas who share his vision of a new kind of village. Then, they'd have to find a suitable location, perhaps a place rich in natural resources but also defensible. Naruto would use his charisma and his own experiences in Konoha to create rules that are fair and inclusive. For example, he could ensure that all ninja clans, no matter how small, have a voice in village matters. And he'd probably want to establish a ninja academy that focuses not only on combat skills but also on things like diplomacy and understanding different cultures among ninjas.
In the sasunaru context, perhaps a rift between Sasuke and Naruto makes Naruto feel like he needs to leave to sort out his feelings. He might think that staying in the village constantly reminded of Sasuke and their complicated relationship is too much to bear, so he leaves to find his own path and hopefully reconcile with Sasuke on his own terms.