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.
A possible plotline could be that a new enemy targets Kakashi because of his distinct face features, believing there is some hidden power in his appearance. Naruto then has to protect Kakashi. Another plotline could be a magical spell that gets cast on Kakashi's face, changing his expressions uncontrollably, and Naruto tries to find a way to break the spell.
One great fanfic could be one that focuses on Kakashi's face and how Naruto is inspired by his calm and confident expressions. Maybe it shows Naruto trying to imitate Kakashi's facial expressions during a tough battle to boost his own morale.
One interesting concept could be a story where Naruto has to draw Kakashi's face as part of a training exercise. He struggles to capture the mystery and calmness in Kakashi's eyes. Another could be a time - travel fanfic where Kakashi's face in his younger days is a key element in changing the future of the ninja world. For example, someone from the future sees Kakashi's face in an old photo and realizes a connection to a major event that needs to be altered.
Some fanfictions might introduce new, original villains. For example, a dark sorcerer - type character who wants to use Naruto's unique chakra for his own evil purposes. Kakashi has to face this sorcerer and his minions to keep Naruto out of harm's way.
Well, for starters, Kakashi has to deal with Naruto's hyperactive and sometimes reckless behavior. Naruto often acts without thinking, which could put him in danger. Sasuke, with his complex emotions and the burden of his clan's massacre, is also a handful. Kakashi needs to make Sasuke understand that revenge is not the only way forward. Additionally, Kakashi has to ensure that both Naruto and Sasuke get along well with each other. There could be rivalry between them, and Kakashi has to manage that in a way that doesn't harm their relationship or their growth as ninjas.
In the fanfiction, Kakashi might face the challenge of Naruto's insecurities. Naruto has always had a complex about being alone and not having a family. Kakashi would have to find ways to make Naruto feel secure and loved. Also, dealing with Naruto's powerful and sometimes uncontrollable chakra could be a problem. Kakashi would need to train Naruto to master his chakra properly without it overwhelming him.
He might become more reserved. Kakashi is often a bit aloof and mysterious, and wearing the face mask like him could make Naruto act in a more subdued way, not showing his emotions as openly as he usually does.
One of the main challenges is time management. The Chunin Exams are approaching fast, and Naruto has a lot to learn. He has to quickly grasp complex ninja techniques that Kakashi is teaching. Another biggie is dealing with his own impatience. Naruto is eager to prove himself and might rush through the training, not fully absorbing the lessons.