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, 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.
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.
Well, one possibility could be that Naruto gets a new arm with some special abilities. Maybe it was given to him by a powerful ally or he discovers an ancient artifact that attaches to him as a new arm. This new arm could have powers like enhanced strength for his ninja techniques or the ability to sense danger more acutely.
One possibility could be the power to control time. In fanfiction, this could be used in various ways, like going back to prevent tragedies or to gain more time to train. It would give Naruto a huge advantage in battles and in his quest to protect the village.
In the fanfiction, Naruto's new sensei could be a mysterious figure who has ties to the Sage of Six Paths. This person might have been waiting for the right time to appear and train Naruto. They could possess knowledge about the origin of chakra and how to fully unlock Naruto's power. It could also be someone who was once an enemy but has now changed their ways and wants to use their skills to train Naruto for the greater good of the ninja world.
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 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.
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.
In this fanfiction, Naruto might learn new and unique jutsus. His new sensei could have a different teaching style, perhaps more strict or more creative, which would greatly impact Naruto's growth as a ninja.