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.
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.
Well, perhaps the new arm comes from some kind of ancient and mysterious source. It could be an arm that was once part of a powerful ninja from the past. This new arm could have a mind of its own, at first causing Naruto some trouble as he tries to control it, but eventually they work in harmony and it gives Naruto an edge in battles against tough enemies.
Well, if we look at the 'Naruto gets his parents back fanfiction', Naruto's character development might take a different turn. Instead of relying solely on his friends and mentors like Iruka and Kakashi, he'd have his parents' direct guidance. His father, Minato, could train him in advanced ninjutsu. His mother, Kushina, could give him all the motherly love and care he was deprived of. This would likely make Naruto more confident and perhaps a bit more relaxed as he wouldn't have that deep - seated longing for family anymore.
Another possibility is that Naruto and Yang have a special bond. This bond could be formed through shared experiences, like they both went through a difficult training together or survived a life - threatening situation. Because of this special connection, Naruto feels indebted to Yang or simply values Yang's life more than his own arm.
One possible start for the 'Naruto gets his parents back fanfiction' is that there is a discovery in an ancient scroll. The scroll contains a spell that can bring the dead back under certain conditions. Naruto, being the curious and determined ninja he is, decides to try it out. Or perhaps, in a different dimension, Naruto's parents are still alive and through some strange portal or rift, they are able to return to the Naruto world.
Maybe he starts to train with one - armed techniques. He could learn from other one - armed ninjas if there are any in the fanfiction world. His friends would also play a big role. They might encourage him to keep going or help him in his training.
Naruto might become more cautious. Losing an arm would make him realize his vulnerability more. He would probably be more strategic in battles, relying less on brute force and more on his intelligence and remaining ninja skills.
In the fanfic, Naruto's character could also become more introverted as he copes with the loss of his arm. He might need time alone to come to terms with his new reality. However, this solitude could also give him time to reflect on his life and his ninja path. He may emerge from this period with a new sense of purpose, perhaps focusing more on non - combat aspects of being a ninja, like diplomacy or teaching. This would be a significant shift from his previous more action - oriented self.
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.