Maybe Kakashi could be the one. He's always been Naruto's mentor in many ways. He has the knowledge and skills to train Naruto for such an unusual task. He could use his Sharingan to analyze the fanfiction and teach Naruto how to spot the dangerous elements within it.
Maybe for his own gain. Kakuzu might see potential in Naruto and think that by training him, he can use Naruto for some future plan. For instance, if Naruto becomes powerful enough, Kakuzu could use him to take on bigger enemies or get access to certain resources.
Yoda would probably start by teaching Naruto about self - control and focus, which are key aspects of Jedi training. He might have Naruto meditate to better sense and control his chakra.
Another aspect of the training could be mental discipline. Hayate may introduce Naruto to meditation - like techniques to help him focus his chakra more effectively. He'd also likely teach Naruto how to keep a calm head in the heat of battle, sharing his own experiences to illustrate the importance of mental fortitude.
The plot could revolve around Naruto seeking to master a new form of ninjutsu. Temari and Yugito decide to help him. Temari uses her skills with wind manipulation to create obstacles for Naruto to overcome. Yugito, with her knowledge of the tailed beasts' chakra, helps Naruto understand and control his own chakra better. As they train, they face challenges like unexpected enemy attacks during the training sessions, which forces them to work together and adapt their training methods.
Naruto might be initially very disappointed. He has always looked up to Jiraiya and expected to be trained by him. So, when faced with the refusal, he could be heartbroken and feel like he has lost a great opportunity.
Another option could be Itachi. Although Itachi is from a different village and has his own complex situation, in a fanfiction scenario, he could secretly train Naruto. Itachi has extremely powerful jutsus and a deep understanding of the Sharingan. Training under him would give Naruto a very different perspective on power and combat, not to mention some insights into the Uchiha clan.
Naruto might be initially very disappointed. He has always looked up to Kakashi as a strong and cool ninja, and being refused by him would be a big blow. He could become a bit sulky and withdrawn, feeling like he's not good enough.
Well, Kurama might train Naruto by teaching him how to better control and access chakra. Since Kurama has a vast amount of chakra, it could show Naruto unique ways to harness and use it. For example, it could guide Naruto in creating more powerful and complex jutsu by using their combined chakra in a more efficient way.