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.
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.
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.
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.
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, 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.