One popular fanfiction concept could be Deku proving his worth without a quirk in the Bakudeku relationship. Maybe he uses his intelligence and strategic thinking to assist Bakugo in battles. For example, he could analyze opponents' weaknesses and come up with battle plans that Bakugo can execute. This would show that even without a quirk, Deku is an important part of their duo.
In fanfiction, a quirkless Deku often has to rely more on his wits. Canon Deku got One For All, but in fanfic, he has to find other ways to be strong. For example, he might study different fighting styles more intensively.
A common theme is the underdog story. Deku, being quirkless, is the ultimate underdog. He has to fight against the odds and the prejudice of a world that values quirks highly. This makes for a very compelling story as he overcomes these obstacles, often with Bakugo by his side or as his motivation.
His relationship with his mother would be extremely strained. She might be in denial about her son becoming a villain and try desperately to bring him back to the light. With his former friends like Bakugo, it would turn into a bitter rivalry. Bakugo, who used to bully him, now has to face a much more dangerous and calculating Deku.
One great quirkless Deku fanfiction is 'The Quirkless Hero'. In this story, Deku uses his intelligence and intense training to outwit villains. He forms an alliance with other underdog characters. It's really inspiring to see how he overcomes the odds without a quirk.
Deku might be completely shocked at first. Since Bakugou has always been so aggressive towards him, he would never expect such a confession. He might just stand there, mouth agape, not knowing what to say.
Perhaps Deku discovers a dark secret about the origin of quirks and decides that the world would be better off without them. So, he starts a villainous campaign to eradicate quirks. He could form an alliance with other outcasts who have been mistreated due to their lack of quirks or abnormal quirks.
He might cry a lot. Deku is very emotional, and being cheated on would be a very painful experience for him. He could also get angry, which is rare for him, but this situation would be so unjust that he might not be able to hold it in. He'd probably start to question his self - worth too, thinking that there was something wrong with him that made his partner cheat.
Deku might be initially shocked. He's used to Bakugou's aggression, but an actual injury is different. He could be in pain and disbelief at first. Then, he might try to downplay it, especially if he doesn't want to cause trouble for Bakugou, because Deku is generally a very forgiving character.
Well, some might be really amazed. They could see a new side of Deku that they never knew before. His singing might show a different level of confidence or a hidden talent, so they would probably cheer him on.