Well, it could be about how Bella copes with the pain and inconvenience of a broken arm. Maybe she has to rely on others more, like her friends or family, to do simple tasks.
Her friends could be main characters. They might be the ones helping her out, like bringing her schoolwork when she's unable to go to school because of her broken arm.
One great fanfiction with this theme could be 'Hermione's Close Call' where Fred is constantly by Hermione's side after she has a near - arm - breaking accident during a Quidditch match gone wrong. It shows his concern in a really sweet way.
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.
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.
Bakugou could use his words. He might tell Deku that he believes in him. For example, he could say 'Deku, you may be a nerd but you're a strong nerd.' It's a backhanded compliment but it shows he has faith in Deku.