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, 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.
The robot arm often serves as a symbol of change and growth in these fanfictions. For example, Naruto might have to adapt to this new limb, which could represent his evolution as a character. He has to learn new fighting styles and ways of interacting with the world around him. Also, the robot arm can have unique powers that are different from traditional ninjutsu, like the ability to interface with machines or have super - strength. This opens up new plot possibilities and challenges for the characters.
It could have enhanced strength. Since Naruto is already quite strong, the fake arm might be designed in the fanfiction to multiply his physical power, allowing him to punch even harder or hold heavier objects.
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.
In the context of the fanfic, it could also impact his relationships. His friends might be overly protective, which Naruto may not like. Or, he could inspire others with his determination to keep going despite his injury. It might also change how he is perceived in the ninja world. Some may see him as weakened, while others could view him as even more of a hero for his ability to persevere.
One possible plotline could be Naruto's struggle to adapt to his new disability. He might have to relearn his ninja skills, like how to fight and perform jutsus with only one arm. This could lead to him developing new and unique techniques that rely more on his chakra control and his legs for mobility.
One great 'naruto robot arm fanfiction' is 'The Mechanical Ninja'. In it, Naruto discovers a robotic arm with unique powers that he has to learn to control. It's a really exciting read as it combines the traditional Naruto elements with cool robotic features.
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.