It could be Orochimaru. Since Anko has a history with Orochimaru, and Nariko being Anko's apprentice, Orochimaru might see Nariko as a threat or a tool to get to Anko. He could try to manipulate Nariko into doing his bidding or try to steal her newfound skills.
Well, it could be about Naruto's transformation into Nariko and her apprenticeship under Anko. Maybe it involves Nariko learning Anko's unique jutsu, like the Snake related techniques. It could also explore how this new identity changes her relationships with other characters in the Naruto world.
It could be a rogue ninja from a rival village who is after the apprentice's special abilities. This rogue ninja might see the apprentice as a threat or a tool to achieve their own evil goals.
Perhaps a strict teacher who doesn't like Naruto's unorthodox ways and Anko's rebellious attitude. The teacher might try to split them up and prevent them from participating in school activities together. For example, if they want to enter a school competition as a team, the teacher could create obstacles.
Perhaps a rogue ninja from another village who sees Naruto as a threat. This ninja could target Anko and Naruto during their training. He might be jealous of the Hidden Leaf's strength and think that if he can stop Naruto from getting stronger under Anko's teaching, he can weaken the Leaf Village. He could be a skilled fighter with unique abilities that pose a real challenge to Naruto and Anko.
It could be a rogue ninja from a hidden village who has a vendetta against Konoha. This ninja might target Kurenai and Anko specifically because of their skills and knowledge, and Naruto gets in the way while trying to protect them.
Obviously, Naruto and Anko would be the main characters. Naruto, the energetic and determined ninja, and Anko, the somewhat eccentric and skilled kunoichi.
The apprentice could be a new genin who shows great potential in the field of intelligence gathering. Maybe someone with a sharp mind and an ability to stay calm under pressure, like Shikamaru. He has the intelligence and strategic thinking that would make him a good candidate for Ibiki's apprentice.
One popular 'Naruto actually Nariko' fanfic could be 'Nariko's Hidden Power'. It often explores a different take on Naruto's character if he were Nariko, with unique plotlines that involve her interacting with the original Naruto characters in new ways. Another might be 'Nariko in the Ninja World', which delves into how Nariko would handle the various challenges and political intrigue within the Hidden Leaf Village.
Well, it could be about Naruto learning some unique jutsu from Anko. Maybe she teaches him her snake - related techniques. Since Anko has a rather wild and unorthodox style, Naruto might grow in ways that are different from his training with Kakashi or Jiraiya.
It could be an original character created specifically for the fanfic. Maybe a young ninja with a unique kekkei genkai that Kakashi sees potential in.