Sasuke could be Naruto's teacher. Since Sasuke is a master of the Chidori, he might decide to teach Naruto as a way to make Naruto a more formidable ally. They have a complex relationship, and this could be a way to strengthen their bond.
Kakashi could also be Naruto's teacher. He was the one who originally developed and used the Chidori. He might see potential in Naruto using this jutsu and decide to pass on his knowledge, especially if there is a great threat that requires Naruto to have more powerful offensive techniques.
In a more creative fanfic, an ancient ninja who was the originator of the Chidori technique could appear. This ninja could be brought back to life through some sort of magic or jutsu. He sees Naruto's pure heart and decides to pass on the Chidori knowledge to him, believing Naruto will use it for good.
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 Sasuke teaches him the Chidori in a moment of camaraderie. And for the Rasengan, it could be an older and wiser Naruto from the future who travels back in time to teach his younger self.
In a fanfiction like this, the plot might involve Naruto struggling to master both techniques. He could start by observing Sasuke use the Chidori and then try to incorporate its lightning - based power into his own jutsu. For the Rasengan, he might have new insights while training, perhaps finding a way to combine it with the Chidori for an even more powerful attack. Along the way, he would face challenges like controlling the chakra flow for both techniques and dealing with opponents who are also after these powerful jutsus.
One possibility could be Kakashi. Since Kakashi is already a very versatile ninja with skills in many areas including some swordsmanship, he could teach Naruto kenjutsu. He has the patience and the knowledge to pass on the skills.
It could be Hinata. Since she is a master of the Gentle Fist in the Naruto universe, she would be a logical choice to teach Naruto. She has a deep understanding of the technique and also has a close relationship with Naruto, which would make for an interesting dynamic in the fanfiction.
One possible plot could be that Naruto discovers the Chidori technique while training in a secret area. Maybe he stumbles upon some old scrolls left by Kakashi. He then has to train hard to master it, facing difficulties like controlling the lightning chakra. As he progresses, he combines it with his own unique Rasengan, creating a new and powerful jutsu.
One possible scenario could be that Naruto learns the Chidori from Sasuke in a moment of truce. Sasuke, realizing Naruto's determination to become stronger, decides to teach him this powerful jutsu. Another could be that Naruto discovers a scroll that contains the secrets of the Chidori and through his own hard work and study, he manages to master it. Or perhaps, a new character appears who has knowledge of the Chidori and trains Naruto in it as part of a greater plan to defeat a common enemy.
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.