One common plot point could be the character with the demon arm struggling with its power. For example, they might have a hard time controlling it during battles, which could lead to some unexpected and dangerous situations. Another plot point could be the origin of the demon arm. Maybe it was a curse from an ancient evil or a result of some forbidden jutsu experiment. Also, the demon arm might attract other dark forces, creating more conflicts in the story as the character has to deal with not only the arm's power but also those who seek to use it for their own malicious purposes.
The demon arm can change a character's role in terms of their relationships. It could drive a wedge between them and their friends if they start to fear the power of the arm. On the other hand, it might attract new allies who are interested in the arm's power or want to help the character control it. In battle, the character with the demon arm might take on a more offensive or even a tank - like role, using the arm's strength to protect their comrades or lead the charge against enemies.
Another plot device is the mystery surrounding the origin of the demon arm. In many fanfictions, the arm's origin is not clear at first. It could be related to an ancient curse, a long - forgotten experiment, or a connection to a powerful and malevolent being from Naruto's world's past. Writers use this mystery to keep readers hooked as Naruto tries to uncover the truth about his arm.
There is a fanfiction where Naruto gets a demon arm after a strange encounter in an ancient temple. The story delves into how he has to hide this new appendage from his friends at first because he doesn't understand it himself. It also has great action scenes where he learns to fight with the added strength and abilities the demon arm provides. Overall, it's a unique take on the Naruto universe.
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.
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.
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.