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.
One possible fanfiction could be about Naruto's right arm gaining a new and mysterious power. Maybe it starts to glow with an unknown chakra, and he has to figure out how to control it without losing himself. It could be a power that has a connection to an ancient ninja clan's secret technique.
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.
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.
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.
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.
Naruto might develop a deeper understanding of sacrifice. His lost arm is a physical reminder of the risks he takes in battle. This could lead to him being more cautious while still maintaining his fearless attitude when it comes to protecting what he loves. He also may become more empathetic towards others who have physical disabilities or losses.
His right arm could get a cool tattoo that gives him extra powers. For instance, a tattoo that allows him to summon a special type of animal or use a unique jutsu.