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.
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.
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.
Well, in some 'naruto loses arm fanfiction', it might be a result of a forbidden jutsu gone wrong. Naruto might be exploring a new and dangerous form of power, and the price is his arm. This would then set off a chain of events where he has to find a way to either get his arm back or adapt. He could also face discrimination from some in the ninja community who see him as weakened, but he'd prove them wrong by his unwavering spirit.
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.
One possible aspect of Finn losing his arm in fanfiction could be how it changes his physical capabilities. He might have to adapt his fighting style, perhaps relying more on his legs or using his remaining arm in more creative ways. For example, he could learn to use his sword one - handed in a whole new way that surprises his enemies.
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.
It could be an old enemy who takes advantage of Percy's weakened state. For example, Kronos might see this as an opportunity to finally defeat Percy and take over Olympus. Since Percy has lost an arm, Kronos might think he's more vulnerable and launch a new attack.
One possible storyline could be Percy's journey of learning to adapt to life with one arm. He might struggle at first with simple tasks like getting dressed or holding a weapon. Another could be him facing his enemies who now see him as weakened due to the loss of his arm, but he proves them wrong with his newfound determination and creative combat techniques.