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.
Perhaps Naruto quits Team 7 in the fanfiction because of a betrayal. He might find out that one of his teammates has been secretly working against him or has lied to him about something really important. This could shatter his trust in the team and make him want to leave. Or it could be due to an accumulation of small incidents. Like constantly being the one to get in trouble for the team's mistakes, or not having his opinions heard when it comes to team strategies. Over time, these little things can build up and make him feel that he's better off on his own.
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.
If Naruto quits Team 7 in the fanfiction, it could introduce a whole new set of storylines. There could be a search for Naruto by Team 7, which would involve them exploring new places and meeting new characters. The emotional impact on the other characters would also be significant. Sakura might feel guilty, thinking she could have done something to prevent Naruto from leaving. Sasuke might be more conflicted about his own goals, as Naruto was always a driving force in his life. And Kakashi would have to deal with the failure of keeping the team together, which could lead to his own character development in unexpected ways.
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.
Well, in such a fanfiction, perhaps Naruto has just lost Sasuke again and is deeply depressed. The ANBU, seeing his skills and his need for a distraction, recruit him. He starts off being a bit of a misfit in ANBU, not following all the rules because of his independent nature. But as he continues, he starts to understand the importance of their work. He might even form new bonds with his ANBU comrades, which slowly help him come out of his depression.
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.