Naruto leaving Konoha after bringing Sasuke back in a fanfic could be the result of various reasons. Maybe he's disillusioned with the village's politics or wants to explore the world on his own terms. It could also be that some outside force lures him away for a greater destiny.
Well, it could be that Naruto leaves because he realizes that Konoha's ways don't align with his ideals anymore after that intense mission. Or perhaps there are hidden threats that force him to go away for the greater good.
Well, it could go in many directions. Maybe Naruto starts a quest to find a way to bring peace to the ninja world on his own terms. Or perhaps he has to deal with the consequences of leaving Konoha and tries to prove his intentions were pure.
Maybe Naruto leaves because he discovers some deep-seated secrets about Konoha that he can't accept. Or it could be that his relationship with Sasuke changes his perspective and he needs to find a new place to grow and evolve.
He might face new enemies and challenges outside Konoha, having to prove himself independently without the village's support.
This type of fanfiction concept is really out of the ordinary. Naruto's character is built on his values of friendship, loyalty, and his dream of becoming Hokage. Killing Sasuke would mean betraying all of that. However, in a fanfiction, the author might be exploring the 'what - if' scenario. For example, what if Sasuke's actions against Konoha were so heinous that Naruto felt he had no choice? But still, it's a far - fetched and very different view from the original Naruto we know.
Perhaps they are forced to leave due to false accusations or being framed for something they didn't do. This could create a situation where they feel they have no choice but to leave Konoha in order to clear their names and also protect the village from the real threats that might be lurking in the shadows.
A possible plotline is that Sasuke is being held captive in a different dimension. Naruto has to find a way to enter this dimension. He meets some strange creatures there who can either help or hinder his mission. Naruto uses his skills and wits to overcome the obstacles. Once he finds Sasuke, Sasuke is initially resistant to coming back. Naruto then has to convince him by reminding him of their dreams when they were kids, like changing the shinobi world for the better. Naruto's unwavering belief in Sasuke eventually breaks through Sasuke's defenses and they return together.
In Shikanaru's fanfic, Naruto might leave Konoha because he feels restricted by the village's rules and traditions. He has always been a bit of an outcast, and leaving could be his way of finding true freedom. The story could follow his adventures in different lands, his attempts to build a new life, and his constant internal struggle between missing his old friends in Konoha and his newfound independence.
Often, a theme is the breakdown of the Naruto - Sasuke relationship. Since Naruto leaves Konoha, it's a huge shift. And the Sasuke bashing can stem from the idea that Sasuke didn't do enough to support Naruto or keep him in Konoha. Maybe Sasuke is shown as being too prideful or cold - hearted in these fanfics. Another theme could be Naruto's search for a new purpose outside of Konoha, away from Sasuke's shadow.
Sakura would probably be very worried. She has always cared about Naruto deep down, so she might be constantly trying to find a way to bring him back. Kakashi, on the other hand, would likely understand Naruto's reasons to some extent. He might be sad but also respect Naruto's decision. As for Sasuke, if he is in the village at the time, he could be conflicted. He and Naruto have a complex relationship, and Naruto's leaving might make him realize how much Naruto actually means to him.
Perhaps they leave because they discover a new threat that the ninja villages are not taking seriously. Naruto and Sasuke decide to take matters into their own hands. They travel to different places, gathering information and allies. Their relationship also evolves during this time, from just comrades to true brothers who understand each other completely.