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.
He might have used magic prosthetics. With the help of his friends like Hermione, who is brilliant at magic research, they could have found a way to create a magical arm that functions almost like a real one.
Well, he might face difficulties in combat. Without an arm, his physical attacks could be less powerful and his balance might be off. He would probably have to rely more on his magic and learn new ways to fight.
One possible plotline could be Naruto having to relearn all his ninja skills with only one arm. He might struggle at first but then discover new ways to perform jutsus, like using his legs more for balance and chakra control. Another could be his friends trying to find a way to restore his arm, leading them on a dangerous journey to find a rare artifact or a powerful healer.
Naruto might become more introspective. He has always been action - oriented, but losing his arm could force him to think more about his actions and their consequences. For example, he may be more cautious when engaging in battles.
Well, in such a fanfic, perhaps Archer sacrifices his arm while saving Arturia from a great danger. Maybe it could be a powerful enemy attack and Archer jumps in the way, taking the brunt of it on his arm to protect Arturia. This would be a very heroic and selfless act on Archer's part.
One possible plotline could be Naruto's struggle to adapt to his new disability. He might have to relearn his ninja skills, like how to fight and perform jutsus with only one arm. This could lead to him developing new and unique techniques that rely more on his chakra control and his legs for mobility.
One possible plot development could be Naruto having to relearn his ninja skills with only one arm. He might struggle at first, but then discover new ways to use his remaining arm in combination with his chakra control. For example, he could focus more on using his legs for powerful kicks and use his one arm mainly for balance and quick jabs of kunai or shuriken. Another aspect could be the psychological impact. Naruto might feel a sense of loss and inadequacy, which could lead to some self - discovery as he overcomes these feelings and becomes stronger in the end.