Well, without having read the 'harry loses crystal arm fanfic', it's hard to say precisely. But it might involve a story where Harry has this unique crystal arm, perhaps it was a gift or the result of some kind of experiment. Losing it could be a central event that sets off a chain of other events. It could lead to him on a quest to get it back, or it could make him face some inner demons as he has to adjust to life without this special part of himself.
You could try searching on popular fanfic websites like Archive of Our Own (AO3) or Fanfiction.net. Just type in the exact title 'harry loses crystal arm' in their search bars and see if it pops up.
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.
One possible plot could be Naruto losing his arm during a battle with a powerful new enemy. He then has to struggle with not only the physical loss but also the emotional and mental toll it takes on him as a ninja. He might have to learn new fighting techniques that don't rely on two arms.
The main antagonist could be an old enemy who has resurfaced and is more powerful than ever. This enemy targets Naruto specifically and manages to injure him so severely that he loses his arm. For example, Orochimaru could return with new and more deadly powers.
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.
It could be that Archer's loss of an arm makes him more determined. He might start to train harder with his remaining abilities, and in the process, discover new skills. This could also make him more respected among his comrades. Arturia, on the other hand, might start to see Archer in a new light, not just as a fellow fighter but as someone she deeply cares about on an emotional level.
One plotline could be Bucky's struggle to regain his combat skills with a new prosthetic arm. He might face challenges in getting used to its functionality and power. Another could be his search for the people responsible for his arm loss, leading to a revenge - driven storyline. Or perhaps he has to deal with the psychological impact, like nightmares or flashbacks, which could be a major part of the plot as he tries to overcome them.
In the fanfic, Naruto's character could also become more introverted as he copes with the loss of his arm. He might need time alone to come to terms with his new reality. However, this solitude could also give him time to reflect on his life and his ninja path. He may emerge from this period with a new sense of purpose, perhaps focusing more on non - combat aspects of being a ninja, like diplomacy or teaching. This would be a significant shift from his previous more action - oriented self.
It could be a new and powerful rogue ninja who manages to overpower Naruto and cause him to lose his arm. Maybe this antagonist has some sort of vendetta against Naruto or the Hidden Leaf village.