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.
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.
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.
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.
Well, in some fanfictions, he might get it from a skilled ninja prosthetics maker. This maker could be someone who specializes in creating artificial limbs for ninjas who have lost their arms in battle.
Naruto would need to train harder to make up for the lack of an arm. He could develop new jutsus or modify existing ones. Maybe he would start using more wind - based attacks that can be controlled with just one arm and his chakra control. Also, his friends might help him by creating prosthetics or assisting him in battles.
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.
Naruto may become more cautious. With a prosthetic arm, he realizes his vulnerability more. He might start to think more strategically before jumping into battles. For example, in a fight, he won't just rely on his brute strength but use the unique features of his prosthetic arm to his advantage.
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.