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.
Perhaps in the fanfic, Naruto loses his arm due to a curse or a forbidden jutsu gone wrong. His friends, like Sasuke, then try to find a way to either get him a new arm or help him come to terms with the loss. It could also show Naruto's journey to adapt to his new disability and still be a great ninja, inspiring the younger generation in the process.
Another scenario might be that Naruto sacrifices his arm to save someone he cares about deeply. He might have used his arm to block a fatal attack aimed at a friend or a loved one, showing his selfless nature even in a fan - created story.
Naruto might become more cautious. Losing an arm would make him realize his vulnerability more. He would probably be more strategic in battles, relying less on brute force and more on his intelligence and remaining ninja skills.
One possible scenario could be Naruto having to relearn his ninja skills with only one arm. He might struggle at first but then find new ways to perform jutsus. For example, he could modify his Rasengan technique to be more one - handed friendly.
Naruto might develop a deeper understanding of sacrifice. His lost arm is a physical reminder of the risks he takes in battle. This could lead to him being more cautious while still maintaining his fearless attitude when it comes to protecting what he loves. He also may become more empathetic towards others who have physical disabilities or losses.
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.