There's a possibility that it's a spirit or a deity in the world of bending. This entity is impressed by Naruto's tenacity and chooses to guide him on his journey of learning bending.
A traveling nen master could be Naruto's mentor. This master could be on a journey to spread the knowledge of nen. He crosses paths with Naruto and decides to teach him. The master might have a strict teaching style, pushing Naruto to his limits, but also guiding him carefully through the complex world of nen.
The fanfiction might be about Naruto accidentally stumbling upon a ancient scroll that contains the secrets of bending. He decides to learn it out of curiosity. He begins with fire bending, which is a challenge as it's very different from his chakra - based fire techniques. But with his ninja training and unwavering will, he manages to figure out how to control the flames in the new way of bending.
Kakashi could be a great mentor. Since he was Minato's student, he has first - hand knowledge of Minato's techniques. He could teach Naruto the fundamentals of the Yellow Flash's fighting style, like how to control chakra for high - speed movement.
It could be Kakashi. Since he's already Naruto's teacher and he knows both techniques. He could guide Naruto in understanding the intricacies of the Chidori, like how to form the lightning - chakra in his hand, and for the Rasengan, he could give more advanced tips on chakra control.
Maybe a mysterious traveler from a far - off land. This traveler has mastered many unique jutsus including Dust Release. He crosses paths with Naruto during one of Naruto's solo adventures and takes him under his wing after seeing Naruto's eagerness to learn new techniques.
It could be Kakashi. Even though in the original story he doesn't teach Naruto the Rasengan, in a fanfic he might have some hidden knowledge about the technique. Maybe he was secretly studying it himself and saw Naruto's eagerness to learn new jutsu, so he decided to share his knowledge and help Naruto learn the Rasengan earlier.
He might learn a time - traveling jutsu. This would be really cool as it could allow him to go back and change certain events in the Naruto world, or gain knowledge from the past to use in the present battles.
In this fanfiction, Kakashi could also be Naruto's mentor in magecraft. Kakashi is known for his adaptability. He could start to study mage powers along with Naruto. They could learn together about new forms of magic, and Kakashi could use his combat experience to help Naruto understand how to use his mage powers in battle situations.
It could be an old hermit who has been living in seclusion for years. This hermit has mastered the Goken and sees potential in Naruto. So, he decides to teach Naruto this powerful technique.
Sakura could be the one teaching him. She's known for being intelligent and having good domestic skills. She might start by showing Naruto the basics in the kitchen, like how to hold a knife properly and how to chop vegetables evenly.