One popular plotline could be them facing a new, powerful enemy together. Since they have a brother - like bond in this fanfiction concept, they would rely on each other's strengths. Naruto's determination and his ability to never give up would inspire Kakashi, while Kakashi's experience and wisdom would guide Naruto.
A third plotline could be centered around them training together. Kakashi could teach Naruto some new and unique jutsu that he has developed over the years. In return, Naruto could show Kakashi new ways of looking at things, like his unorthodox fighting style which is based on his creativity and willpower.
One possible plotline could be Kakashi teaching Naruto some super - advanced jutsu that Naruto then uses to surprise his friends in a sparring match. For example, Kakashi might teach Naruto a special lightning - style jutsu that Naruto masters and combines with his Rasengan.
In some fanfictions, there could be a plot where Naruto and Kakashi get transported to a different dimension. In this strange new world, they have to find their way back home while facing various challenges. Along the way, they learn more about their own strengths and weaknesses and how their partnership is crucial.
Perhaps a plot where Naruto's determination in the face of some sort of 'heat' - like a difficult situation in the village, and Kakashi is there to guide him. It could involve Naruto learning new jutsus under Kakashi's tutelage while also dealing with his own growing pains.
Well, in some 'kakashi teaches naruto fanfiction', there might be a plotline where Kakashi teaches Naruto how to better control his chakra. This could involve some really intense training sessions deep in the forest. Also, Kakashi could be teaching Naruto about the art of stealth and infiltration, skills that Naruto doesn't have much experience with but are crucial for a ninja.
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.
Another plotline could be the relationship between Naruto and Kakashi growing deeper. Maybe Naruto faces a personal crisis and Kakashi has to use his own experiences to guide Naruto through it. This could show a more emotional side to their master - apprentice relationship.
One possible plotline could be that Kakashi arrives just in time to save Naruto from a powerful enemy's ambush. He uses his Sharingan to predict the enemy's moves and counter them effectively, allowing Naruto to escape unharmed.
Perhaps a plotline where Kakashi and Iruka have to protect Naruto from the Akatsuki. They train Naruto intensively to prepare him for any potential attacks. At the same time, they try to keep Naruto's identity as the Jinchuriki of the Nine - Tails a secret from the Akatsuki members who are after his power.
A plotline could be that there is a powerful new enemy that specifically targets their bonds. This enemy uses illusions and mind - control to try and turn Sasuke against Naruto, or make Sakura distrust Kakashi. They have to fight not only the physical attacks of this enemy but also the mental and emotional turmoil it causes within their group. In the end, they realize the true strength of their relationships and are able to defeat the enemy.
One common plotline could be Kakashi constantly protecting the female Naruto from various threats. He might be overprotective in battles, not allowing her to take too much risk. For example, when facing a powerful enemy, he would step in immediately to shield her.