Even though Orochimaru is a bit of a controversial figure, in a fanfic, he could be Naruto's mentor in a different way. Maybe Naruto tries to learn from Orochimaru's vast knowledge of jutsus, but at the same time, he tries to correct Orochimaru's wrongdoings. This would be an interesting dynamic as Naruto tries to take the good parts of Orochimaru's Sannin - like qualities while avoiding the evil.
Well, if Naruto becomes a Sannin, he would surely gain some amazing new abilities. For one, he could inherit Tsunade's healing abilities. This would be a great addition to his skills as he could not only fight but also heal his comrades on the battlefield. In terms of Jiraiya, Naruto might be able to master more advanced forms of Rasengan, inspired by Jiraiya's own jutsu creativity. And from Orochimaru, he could get a better understanding of the chakra nature and how to manipulate it in more complex ways, like Orochimaru did with his snake - like jutsus.
It could be Jiraiya. He has a really strong bond with Naruto already. He's taught Naruto a lot in the original story too. So in the fanfic, his training could be the most in - depth, covering things like advanced rasengan variations and how to fight against really powerful enemies. His influence on Naruto's character development would also be significant.
Even an older Uchiha like Itachi (in a non - villainous AU perhaps) could be a mentor. The Uchiha are known for their fire - based jutsu. Itachi could teach Naruto the subtleties of fire control, like how to vary the intensity and shape of the flames for different combat situations.
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.
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.
Jiraiya would be the most interesting. He has a wild and carefree personality which could be a great contrast to Naruto's initially lonely and neglected state. Jiraiya is also very knowledgeable about the outside world and has many powerful jutsus, especially those related to the toads. He could take Naruto on adventures, teaching him not just about being a ninja but also about life in general. His connection to the prophecy about the 'Child of Prophecy' could also add an interesting layer to the story, as Naruto might be groomed in a different way to fulfill that prophecy under Jiraiya's guidance.
Perhaps a mysterious figure who has been observing Sakura from the shadows. This person, for some reason, decides to train her to be an assassin. He could have some ulterior motives, like using Sakura for a grander plan, but still teaches her valuable skills along the way.
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.