There's a fanfic where Naruto loses an arm but then trains with an old hermit in the mountains. In this story, he discovers new jutsus that are specifically designed for his one - armed state. It's a really interesting take on his character development and shows his resilience.
One common theme is resilience. Naruto having one arm is a major setback, but in these fanfictions, he always bounces back. For instance, he might struggle at first but then finds ways to be as strong as before.
Sasuke could be a candidate. Given their complex relationship, Sasuke might find a way to give Naruto a new arm as a way to atone for their past differences. He could have discovered some rare medical ninjutsu during his travels or come across a powerful artifact that he uses to create a new arm for Naruto.
One popular theme is redemption. The idea that Naruto, even with one arm, can redeem himself in the eyes of those who doubted him. Another theme is the discovery of new powers. Since he has a handicap, he has to find new ways to fight and that often leads to the creation of unique jutsus.
There's a fanfic where Naruto has one arm and it focuses on his relationship with his friends. They don't pity him but instead, they help him adapt. For example, Sasuke trains with him to improve his one - armed combat skills. It's a great exploration of friendship in the face of adversity.
Maybe Orochimaru. Despite being a villain in the past, he is a master of forbidden jutsus and experiments. He could see Naruto as an interesting subject and give him a new arm with some dark powers, which Naruto would then have to learn to control for the greater good.
Well, it could be that he got it as a gift from some super - advanced ninja technology group in the fanfiction world. Maybe they saw his need for an extra weapon or tool during battles and decided to equip him with a metal arm.
One special ability could be enhanced strength. The metal arm might be able to deliver much more powerful punches than his normal arm, allowing him to break through tough defenses more easily.
There could be a plot where Naruto's friends are overly protective of him at first, but he has to convince them that he can still fight and be useful. Also, the story might focus on how Naruto's relationship with Sasuke changes. Sasuke might see Naruto in a new light and feel a new sense of respect for him because of how he copes with the loss of his arm.
Maybe he was protecting someone in Wave. He's always the self - sacrificing type, so he could have put himself in harm's way to save a villager or a friend.