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 some fanfictions, Naruto might learn Rasengan through the guidance of an alternative mentor. Maybe a powerful shinobi from another village takes him under their wing and shows him different techniques to master the Rasengan. This could add a whole new dynamic to the story and his character development.
Well, aside from Naruto, Orochimaru could be a main character in this fanfiction. He has always been interested in powerful and dark jutsu. He might be the one who either creates or tries to steal the dark Rasengan for his own evil purposes. And then there's Kakashi. He could be involved as Naruto's mentor, trying to guide him away from the dark path that the dark Rasengan might lead him to.
Well, it could be about a dark and twisted version of the Rasengan in the Naruto universe. Maybe it involves Naruto using the Rasengan in a more sinister or unorthodox way, perhaps against his former friends or allies in a story full of conflict and moral ambiguity.
One possible plot could be that Naruto discovers an ancient scroll that has a simplified version of the Rasengan technique. He spends days secretly practicing it in the forest, facing initial failures but gradually getting the hang of it. With his new - found skill, he surprises his friends and even manages to use it in a sparring match against Sasuke, changing the dynamics of their rivalry early on.
The key element is Naruto's determination. He has to be really determined to learn both the Rasengan, a powerful chakra - based attack, and the Hiraishin, a complex space - time ninjutsu. Without that strong will, he wouldn't be able to endure the tough training.
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.
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.
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.