Maybe it's to protect the village. He could discover a threat that he can only face away from the Leaf Village, to keep the danger from directly reaching his home and loved ones. He leaves as a form of self - sacrifice.
One possible reason could be feeling betrayed by the village's secrets and politics. Maybe he discovers some dark underbelly of the Leaf Village that makes him lose faith in it.
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.
A simple reason could be for adventure. Sakura might be tired of the daily routine in the village and yearn for new experiences outside. So she decides to leave to explore the world and test her skills against new opponents.
Another reason might be that Naruto is seeking a greater purpose. In the fanfiction, his 'godlike' state could make him realize that there are threats out there that he can't face while being tied to Konoha. He may leave to protect the world on a larger scale, believing that he can be more effective away from the village.
One reason could be to seek more power. In some fanfictions, Naruto might feel that the Leaf doesn't offer enough resources or training for him to reach his full potential. So he decides to leave to find a place where he can grow stronger.
He might meet Iruka first. Iruka was always like a father figure to Naruto, and it would be a very heartwarming reunion. Iruka would probably be one of the first people to welcome Naruto back with open arms.
Perhaps there have been disputes between the fanfiction community and the official Naruto representatives. For example, if the fanfiction community has been misusing or over - exploiting the Naruto character in a way that goes against the spirit of the original work. This could lead to the decision of asking Naruto to be removed from fanfiction.
Perhaps continuous disappointment. If all his efforts to bring peace and unity were constantly thwarted, like his attempts at forging alliances between different ninja clans being undermined time and again. This build - up of frustration might lead him to snap and decide to leave in search of a new way or place where his ideals could be realized.
The plot could involve Naruto coming back to the Leaf Village to fulfill a prophecy. He may have discovered something during his travels that made him realize he has a crucial role to play in the village's future. Along the way, he could face challenges from within the village, like some people being skeptical of his newfound knowledge or power. But through his determination and the bonds he has with his friends, he overcomes these obstacles and helps the village thrive.
One reason could be a mission. Maybe he gets a top - secret mission that requires him to leave the village under Tsunade's leadership without her direct involvement. So he has to leave without much notice.