Perhaps Jiraiya could return in the fanfiction as a spirit to mentor Naruto in the secret training. Jiraiya could pass on some more of his sage mode secrets or some new toad - related jutsu that he didn't have the chance to teach Naruto before. This would be a great way to honor Jiraiya's character and also give Naruto a new edge in combat.
It could be Kakashi. Since Kakashi has a wealth of knowledge and experience, he might take Naruto on a secret training journey. He could teach Naruto some advanced sharingan - related techniques that Naruto can adapt to his own fighting style, even though he doesn't have the sharingan himself.
There's also a possibility that an unknown and powerful ninja from a far - off land could be the mentor. This ninja might have a completely different view on chakra and ninja training. He could introduce Naruto to concepts like combining natural elements in a way that's not commonly practiced in the Hidden Leaf Village, leading to Naruto developing a truly unique set of skills.
Maybe Dumbledore in a more in - depth way. In the original series, Dumbledore was somewhat mysterious in his teachings. Here, he could really focus on Harry's growth and give him proper training, perhaps in the art of divination and understanding prophecies better.
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.
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.
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.
In some naruto secret training fanfiction, the training could be centered around Naruto's connection to the Nine - Tails. He might learn how to tap into the Nine - Tails' chakra in a more controlled and efficient way without the negative side effects. This could involve some spiritual journey within his own mind - scape where he and the Nine - Tails reach a new level of understanding and cooperation, leading to a huge boost in his power.
It could be Itachi. Even though Itachi has his own complex past, he is highly skilled in Anbu - like operations. He could show Naruto the ropes of the Anbu world, teaching him how to analyze situations quickly and make tough decisions that are often required in Anbu missions.
Iruka could be his mentor. Iruka has always been there for Naruto from the start. He could teach Naruto the basics of swordsmanship, using his knowledge of the ninja arts. Iruka would be patient and understanding of Naruto's blindness, helping him to develop other senses to compensate for his lack of sight.
Maybe Tsunade. She is known for her love of sake. She could have some hidden knowledge of the Drunken Fist. Since she is also a very powerful ninja, she could train Naruto in both the physical and strategic aspects of the Drunken Fist. She could show him how to use his chakra more effectively while performing the drunken - like maneuvers.
There's a possibility that it's a spirit or a ghost of a past master of the drunken fist. Naruto somehow makes contact with this entity, and the spirit decides to pass on its knowledge to Naruto, guiding him through the process of mastering the drunken fist in a rather supernatural way.
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.