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.
Maybe he was protecting someone in Wave. He's always the self - sacrificing type, so he could have put himself in harm's way to save a villager or a friend.
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.
Another possibility is that Naruto and Yang have a special bond. This bond could be formed through shared experiences, like they both went through a difficult training together or survived a life - threatening situation. Because of this special connection, Naruto feels indebted to Yang or simply values Yang's life more than his own arm.
Well, in some fanfictions, it might be due to a dangerous experiment gone wrong. Harry could have been trying to create a new spell or potion to defeat Voldemort once and for all, but something exploded and cost him his arm.
Maybe he starts to train with one - armed techniques. He could learn from other one - armed ninjas if there are any in the fanfiction world. His friends would also play a big role. They might encourage him to keep going or help him in his training.
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.