The metal arm might have some sort of chakra - storing or amplifying ability. Naruto could store his chakra in the arm and then release it in a more concentrated and powerful form, like a super - charged Rasengan that he can launch from the palm of the 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.
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.
For a Naruto character with a metal arm, say it's a new and unknown shinobi. Maybe they were born into a family of blacksmiths who were secretly working on creating advanced weaponry. An accident during the creation of the metal arm led to the young shinobi losing their original arm. But instead of giving up, they decided to use the prototype metal arm. They then trained hard to master its use and became a force to be reckoned with in the Naruto world, using their unique arm to surprise opponents in battles.
Yes, there might be. The Harry Potter fandom is vast and diverse, and fans often come up with creative and unique concepts like this for their fanfictions. There could be a writer who decided to give Harry Potter a metal arm in their story for various reasons, perhaps as a result of a battle injury or some magical - mechanical experiment gone wrong.
One possible character could be Rock Lee. If he had a metal arm, it could enhance his taijutsu. Maybe he got it after a dangerous mission where he lost his original arm. He could use the metal arm to deliver more powerful punches and even incorporate some unique techniques. For example, he could attach weights or blades to it for different combat situations.
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.
The weird arm might enhance Naruto's physical strength. For instance, he could be able to lift much heavier objects or deliver more powerful punches.
One possible plot could be that Harry gets his metal arm during a battle with Death Eaters. The arm could be magically enhanced, with runes inscribed on it that give him new abilities like being able to sense dark magic from a distance. He then has to learn to control these new powers while also dealing with the prejudice from some in the wizarding world who see his metal arm as something 'unnatural' in a magical setting.
His metal arm could come from a cursed object that backfired on him. For example, he might have been trying to disarm a powerful dark magic item and it ended up fusing with his arm, turning it into metal. This would then force him to learn how to control the new power within the metal arm.
In some fanfictions, it could be a gift from a parallel magical dimension. There could be a race in this other dimension that is very advanced in combining magic and metalwork, and they see Harry as a hero worthy of such a gift. The metal arm could then hold the secrets and powers of that dimension, which Harry has to figure out how to use to save the wizarding world.
Well, his metal skin could enhance his defense greatly. Enemies' attacks like kunai stabs or basic jutsu blasts might be deflected or absorbed more easily. It could also add weight to his punches, making his physical attacks more powerful.