Well, in fanfic, the 'weird arm' could be a result of some experimental jutsu gone wrong. Maybe Naruto was involved in a secret mission where he was exposed to some strange chakra that mutated his arm.
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.
Well, in some fanfictions, he might get it from a skilled ninja prosthetics maker. This maker could be someone who specializes in creating artificial limbs for ninjas who have lost their arms in battle.
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.
One common theme is resilience. Naruto having one arm is a major setback, but in these fanfictions, he always bounces back. For instance, he might struggle at first but then finds ways to be as strong as before.
You can try fanfiction.net. It has a large collection of Naruto fanfics, and there's a good chance you'll find some 6 arm Naruto fanfics there. Just use the search function and type in relevant keywords like '6 arm Naruto' and browse through the results.
One popular 6 arm Naruto fanfic might be 'The Six - Armed Ninja'. In this type of fanfic, the concept of Naruto having six arms could bring a whole new dimension to his combat abilities. For example, he could use multiple jutsus simultaneously, like performing Rasengan in two hands while using shadow clones with the other four arms for a coordinated attack. Another fanfic could explore how his friends and the village react to this strange new transformation. Does he face fear or acceptance from his peers?
In the context of the fanfic, it could also impact his relationships. His friends might be overly protective, which Naruto may not like. Or, he could inspire others with his determination to keep going despite his injury. It might also change how he is perceived in the ninja world. Some may see him as weakened, while others could view him as even more of a hero for his ability to persevere.