One possible storyline could be Naruto's struggle to adapt to his new physical state. He might have to relearn all his ninja techniques, now that he's missing an arm. For example, he'd have to find new ways to perform his signature Rasengan. Another could be his journey of self - acceptance, dealing with feelings of inadequacy compared to his former self. He could also face challenges from enemies who see his injury as a weakness and try to take advantage of it.
His combat abilities are severely affected. Naruto is known for his physical combat skills combined with his chakra - based attacks. Without his right arm, his punches and grappling moves are limited. He can't hold his kunai or shuriken as effectively in combat, which reduces his ranged attack options.
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.
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.
One great source for Naruto and Hinata fanfiction could be FanFiction.net. You can search for stories specifically focused on their relationship there. There are often new and interesting takes on their story, from how they first fell in love to their married life in the future.
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.
It could be Orochimaru. He's always been interested in Naruto's power, especially the Nine - Tails within him. He might have come up with a new devious plan to capture Naruto and use his power for his own dark experiments.
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.