Perhaps it delves into Naruto's past in a more detailed way. It could show events that shaped him before we meet him in the main story. For example, his experiences as an orphan and how that really affected his mental state. This can give a whole new dimension to his character.
Perhaps he's trying to protect himself from some kind of danger. There could be an enemy who is looking for him and by covering his face, he can avoid being detected easily. It could also be a part of a new jutsu or power he has discovered where covering his face is necessary for its activation or control.
The plot could be that Naruto gets a special mission to protect Kakashi's face from a group of rogue ninjas who want to steal Kakashi's identity. Naruto has to use his unique skills to fend off the attackers and keep Kakashi's face safe.
No, it is not appropriate at all. Such content involves vulgar and inappropriate themes which go against the positive values of the Naruto series. The Naruto series is mainly about friendship, growth, and perseverance, and fan - created works should also uphold positive and healthy themes.
One interesting idea could be the revelation of Naruto's real face in terms of his family heritage. Maybe the fanfiction delves into secrets about his parents that were not fully explored in the official story. It could show how Naruto discovers these secrets and how they change his perception of himself and his place in the ninja world.
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.
One challenge Naruto might face is Sasuke's own will. Sasuke may be so consumed by his thirst for revenge or his dark path that he doesn't want to be brought back. Naruto has to break through that mental barrier. Another challenge could be the external forces that are influencing Sasuke, like the people or organizations that have turned him to the dark side. Naruto has to deal with them as well.
He could face some of his old rivals like Sasuke. They have a long - standing competitive relationship, and it would be exciting to see them compete again in the Jonin exams.
One challenge could be cultural differences. Naruto is from the ninja world with its own set of values and traditions, while Yasaka is from the youkai world. Another might be the acceptance of their relationship by their respective communities. The ninja might be suspicious of a youkai, and the youkai might not fully trust a human like Naruto at first.