One possibility is that Naruto's skills would be put to extreme use in ANBU's more secretive and dangerous missions. His unique abilities like the Shadow Clone Jutsu could be utilized in stealth and assassination operations, but he might also face a moral dilemma as ANBU often deals with morally grey areas.
Naruto's development could involve him becoming a more calculating shinobi. In ANBU, he has to think several steps ahead. His fighting style might change from his usual straightforward attacks to more strategic and stealthy ones. He would also have to suppress his emotions more, which could lead to some internal turmoil as he struggles to keep his true self hidden.
In a more complex scenario, Naruto could be influenced by an outside force. Let's say an enemy of Konoha uses mind - control or some form of manipulation on Naruto. They make him believe that the ANBU is the enemy of the village and that he needs to take them down. As he betrays the ANBU, he slowly starts to realize that he's been used, but by then he's already in too deep and has to find a way to fix the situation and regain the trust of his friends and the village.
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.
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.
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.
In such a fanfiction, the plot could involve Naruto being constantly overlooked by the village. Fed up with this, he decides to join ANBU to prove his worth. He might face challenges like distrust from his ANBU comrades at first because of his reputation. But as he goes on missions, he shows his skills and determination, gradually earning their respect and changing their perception of him.
Well, in a fanfiction where Naruto joins ANBU, his training would intensify. He'd be trained by some of the toughest ANBU veterans. His skills in combat would become more refined as he'd learn to use weapons like kunai and shuriken with more precision. He may also develop new jutsu that are more suitable for covert operations. His growth as a ninja would be different from the regular path, focusing more on secrecy and assassination if required.
He could become a more solitary and brooding character. In the ANBU, Naruto might use his pain as fuel for his missions. He could start to distance himself from his old friends, as the nature of ANBU work is often secretive and isolating.