You might consider adding Tenten to the team. She has a vast knowledge of weapons which could be handy in various missions. And Kiba, with Akamaru by his side, could offer enhanced senses and tracking abilities for the Anbu team.
Maybe in the fanfiction, the ANBU Team 7 is sent on a mission to infiltrate a hidden organization that is planning to attack Konoha. Naruto uses his shadow clones in creative ways to gather intelligence. Sakura disguises herself as a member of the organization and uses her knowledge of poisons to take out guards silently. Sasuke uses his Sharingan to detect hidden traps and enemies. Along the way, they discover that there is a traitor within Konoha who is feeding information to the enemy.
It could be Orochimaru. He has always been interested in Sasuke and might target ANBU Team 7 to get to him. Orochimaru's experiments and his desire for power make him a formidable and menacing villain. He could also be using some new jutsu or have created more powerful minions to fight against Team 7.
Another plot could involve the Anbu team being tasked with protecting a young and powerful ninja from another village who has valuable knowledge. Along the way, they form an unexpected bond with this ninja and learn new techniques from them.
Sasuke might be a bit jealous at first. He's always been competitive with Naruto, and seeing Naruto in a special and elite unit like ANBU could trigger that jealousy. He might try to prove that he's just as good or better. Sakura, on the other hand, would probably be worried. She cares a lot about Naruto and ANBU missions are often dangerous.
In the ANBU after leaving Team 7, Naruto will be confronted with numerous challenges. Physically, the missions are likely to be more intense, requiring him to further develop his skills and combat abilities. He'll need to learn new stealth techniques and master different types of weapons. Emotionally, he'll face isolation. ANBU work is solitary, and Naruto, who thrived on the camaraderie of Team 7, will have to adjust to being alone most of the time. Additionally, he'll be faced with complex political situations within the ANBU organization. He'll have to navigate through power struggles and different factions, all while trying to stay true to himself and his values.
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.
Perhaps a powerful and unknown shinobi from a far - off land. This shinobi has heard of Team 7's reputation and sees them as a threat to his plans of world domination. He uses his unique and mysterious jutsu to challenge Team 7 at every turn, and they have to figure out his motives and weaknesses to stop him.
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, 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.