The coat might be a key element that drives the plot forward. For example, if the character with the coat is being chased because someone wants the coat, that would create action and conflict.
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.
Losing his arm would be a huge blow to Harry. His friends' reactions would be mixed. Hermione would be practical, trying to find magical solutions or aids for Harry. Ron would be more emotional, showing his support through his presence. Some of his other friends might be a bit afraid or unsure at first. But over time, they would all come to accept Harry as he is now and their relationships would be based on a new level of understanding and support.
There's a visual novel called 'Fate's Twist' where a guy loses his arm due to a magical curse. The curse was cast by an evil sorcerer and has a significant impact on the character's journey and development throughout the story.
The way Oliver Queen loses his arm in comics isn't always the same. Sometimes it's a result of a risky mission gone wrong, or it could be from a confrontation with a villain with unique abilities. It all depends on the particular comic storyline.