Perhaps Kurenai first shows Naruto the real danger that lurks in the shadows of the village. She makes him understand that ANBU is the front line in dealing with these hidden threats. Then she tells him about how his growth as a ninja has led him to a point where he can handle such dangerous and secretive missions. She might also mention that by joining ANBU, he can gain more knowledge and power to protect not just the village but also the world from greater evils. Naruto, being the hero - type, would be swayed by such a grand vision presented by Kurenai.
She might convince him by showing him the long - term benefits for the village. For example, she could tell Naruto that in ANBU, he can prevent threats before they become major problems.
One possible plot could be that Kurenai has some sort of influence on Naruto's decision. Maybe she sees his potential in a different way than others. She could encourage him to join ANBU to better protect the village and its people. Naruto, being influenced by her words and perhaps having a sense of admiration for her, decides to take on the challenge of joining ANBU.
The Anbu sister could be a powerful and mysterious character. In the fanfiction, she might have unique abilities that Naruto is curious about. Their meetings could be filled with tension at first, as the Anbu are often secretive. But as Naruto's friendly nature breaks through, they start to share their stories.
One possible fanfiction could be about Naruto forming an unexpected bond with Kurenai's sister who is in the Anbu. Maybe Naruto stumbles upon her during a mission and they start to work together, gradually learning about each other's pasts and dreams.
For Kurenai, her character could be developed by exploring her relationships with other members of the village more deeply. For example, showing how she mentors the younger generation in addition to her regular duties. In the context of ANBU, she could be given a role where she has to train some of the ANBU recruits. As for Naruto, if he is involved, his time in the ANBU could show his growth in stealth and strategy, which are different from his usual combat style.
Another idea is a story where Naruto joins the ANBU and is paired with Kurenai for a special undercover operation. They have to infiltrate an enemy organization that is suspected of planning an attack on Konoha. Along the way, they learn more about each other's pasts and develop a strong bond as comrades.
In this fanfic, Naruto might face tougher and more secretive missions. He could be trained in new and deadly ninja techniques specific to ANBU. Also, his relationships with other characters might change. For example, he might become more distant from his old friends as ANBU work often requires secrecy.
One challenge would be the secrecy. Naruto is used to being open and honest with his friends, but in ANBU, he has to keep things hidden. Another is the new type of missions. He may not be used to the stealth and assassination - type missions that ANBU often undertakes.
Well, perhaps in this fanfic, the pregnancy could be a result of a special mission or some sort of magic that binds Kurenai to a character. And Naruto, being the hero he is, gets involved to help Kurenai understand and deal with the situation. He could also help her face the reactions of the village, which might be mixed since Kurenai is a respected shinobi.
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.
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.