What could be the story behind 'naruto has a weird arm' in fanfic?

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.

Maybe Naruto discovers that the weird artifact has some special powers, like the ability to open portals to other dimensions.

A possible main antagonist in such a fanfic could be a dark entity that has possessed someone close to Naruto. This entity could be using the host's body to get close to Naruto and then attacks him, resulting in the loss of his arm. Naruto would then have to figure out not only how to deal with his injury but also how to save his friend from the dark entity's control.

The main antagonist could be an old enemy who has resurfaced and is more powerful than ever. This enemy targets Naruto specifically and manages to injure him so severely that he loses his arm. For example, Orochimaru could return with new and more deadly powers.

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.

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, 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.

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, in this fanfiction scenario, perhaps Naruto met Fuu during one of his travels. They might have been on a mission together where they faced a great danger and in the process, their bond grew stronger. Eventually, they fell in love and had a daughter. The daughter could inherit Naruto's strong will and Fuu's unique abilities, if she has any in the fanfiction.

Well, it could be that the mask in the fanfic has some sort of special power or significance. Maybe it's a mask that reveals a hidden identity of a character, like Naruto himself or someone else in the Naruto universe. And this revelation shocks all the other characters.