He might learn a time - traveling jutsu. This would be really cool as it could allow him to go back and change certain events in the Naruto world, or gain knowledge from the past to use in the present battles.
One great Naruto boxing fanfiction is 'The Ring of the Ninja'. It has really interesting fight scenes where Naruto uses his ninja skills in the boxing ring. It's a fresh take on the Naruto universe.
One possible scenario in this fanfiction could be that Naruto, being banished from his village, stumbles upon a magical realm. There, he might meet a mentor who teaches him magic. This could lead to him developing new powers and a whole new identity, different from the ninja he once was. It could also open up opportunities for crossovers with other magical universes in the fanfiction world.
In fanfiction, Naruto learning the Vacuum Jutsu could be really cool. It might give him a new and powerful offensive technique. Maybe he'd learn it from an unexpected source, like an old hermit in a forgotten land. This could potentially change the power dynamics in battles he's in.
Sakura could be the one teaching him. She's known for being intelligent and having good domestic skills. She might start by showing Naruto the basics in the kitchen, like how to hold a knife properly and how to chop vegetables evenly.
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.
It could be an old hermit who has been living in seclusion for years. This hermit has mastered the Goken and sees potential in Naruto. So, he decides to teach Naruto this powerful technique.
One possible plot twist could be that his heritage is linked to an enemy he thought was unrelated. So he has to reevaluate his views on the whole conflict.
The combination of Naruto's unique abilities and the sport of boxing. Naruto has all these cool ninja powers like shadow clones, and seeing how they could be used in a boxing context is really interesting.