Naruto faces a whole new set of challenges. Firstly, the ANBU has a strict code of conduct which he needs to adapt to. The missions are often solo or with very small teams, so he has to rely more on himself rather than his comrades like in Team 7. Also, the enemies he faces in ANBU are often more powerful and cunning. He has to keep his identity a secret, which is a big change from his more open - natured role in Team 7.
Well, his relationship would become quite strained. Sasuke might feel betrayed as Naruto was an important part of their dynamic. Sakura would be confused and sad. She had come to rely on Naruto in many ways. Kakashi, as the sensei, would probably feel like he failed Naruto in some way. Naruto's departure would leave a big gap in the team, and there would be a lot of unspoken emotions.
Well, one major plot change could be in the power dynamics. Since Naruto in Anbu is likely more skilled and experienced. He might take a more dominant role in Team 7's missions. For example, he could quickly assess and handle dangerous situations that the original Team 7 might struggle with. This could also change the relationships within the team. Sasuke might be more competitive towards Naruto, seeing him as an even greater rival. And Sakura might have more admiration or fear towards Naruto because of his Anbu status.
It could also be due to a major disagreement with Kakashi. Maybe Naruto has a different vision for how they should operate as a team or a particular mission that goes against his values, causing him to leave.
In some fanfictions, Naruto might join Anbu because of Sakura to prove himself worthy of her. Maybe he thinks that by becoming an elite Anbu member, he can show her that he's strong and reliable.
Naruto's personality might become more reserved initially. The Anbu experience often makes ninjas keep their emotions in check. So, he might not be as boisterous as the original Naruto. For Sasuke, he could be more on edge around Naruto. He would sense the change in Naruto and feel pressured to keep up. Sakura might become more cautious too. She would see the more serious side of Naruto and might try to be more like him in terms of being focused during missions.
One possible reason could be a sense of being undervalued. Maybe he felt that his teammates didn't fully appreciate his efforts during the Wave mission. For example, if Sasuke got most of the credit, Naruto might feel pushed aside.
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.
Well, Naruto might face a whole new set of challenges. In the ANBU, he could be pushed to his limits physically and mentally. Maybe he'd use his unique skills in stealthier ways compared to his normal fighting style. Also, his relationships with other ANBU members would be interesting to explore, as he's a bit of an outlier with his loud and brash personality.
In this kind of fanfiction, Naruto might face some really tough and secretive missions in ANBU. He could be trained in new and deadly ninja techniques. Maybe he would also form some unexpected alliances with other ANBU members, learning from their different skills and experiences.