It depends on the type of quirk. If it's a power - enhancing quirk, he might gain some unexpected abilities that he has to learn to control. On the other hand, if it's a more harmful quirk like a mind - control quirk, he could be forced to do things against his will. His students would then have to find a way to save him, which could create a really interesting plot in the fanfiction.
He might lose his control over his own quirk for a while. Since his quirk is erasing others' quirks, if he's hit by a strange quirk, it could disrupt his own quirk's function.
Well, in such a fanfiction, Aizawa might suddenly age rapidly. His physical appearance would change, with wrinkles and grey hair appearing. His once-sharp reflexes could be dulled due to the aging of his body. This could put him in a vulnerable position, especially in a world full of villains. And his relationship with his students might also change. They might start to see him more as a frail old man rather than their strong teacher.
Villains could also interact with him in an unexpected way. Some villains might see his weakened state as an opportunity to take him out. However, there could also be villains who respect him and might even offer help, thinking that it's not a fair fight if he's been hit with such a quirk.
In a fanfiction where Deku is hit with a truth quirk, there are many possibilities. Firstly, his interactions with his mentors like All Might would become very different. He might admit his doubts about being a worthy successor, which could either strengthen or strain their relationship. Secondly, his classmates would be shocked by some of the things he reveals. For instance, if he tells them how much pressure he feels to be the best. This could lead to more understanding and support among them, or it could create some jealousy and rivalry depending on how the other characters react. Lastly, Deku's own view of himself would change as he has to face his unfiltered thoughts and feelings.
Deku's character development in this scenario is complex. At first, he may be embarrassed and scared by the things he can't help but say. But as time goes on, he learns to embrace this new state. He realizes that being truthful all the time, even about his weaknesses, can be a strength. His relationships evolve as his friends come to appreciate his honesty. He also grows in terms of his hero identity. Instead of trying to project a perfect image, he shows his true, flawed self, which makes him a more believable and inspiring hero in the fanfiction world.
He could become more empathetic. If the quirk makes him experience something difficult like the pain of others or a disability, it might make him more understanding of those around him who are less powerful or have faced hardships.
He might lose his own quirk temporarily. In some fanfictions, a quirk hit can disrupt a character's own power system.
She might lose her memory. This would be a big change in the story as she'd have to relearn about her past and the people around her. It could lead to some interesting character interactions as others try to help her remember.
In fanfiction, if Izuku was hit by a love quirk, he might start acting in a very affectionate way towards a particular character. For example, he could become overly protective or constantly seek the person's attention. His normal shy and reserved self might be replaced with a more forward and love - stricken version.
Well, if Izuku gets the Hero Killer's quirk in fanfiction, he might face a lot of internal conflict. On one hand, his original goal of being a hero is all about saving people with a smile. But the Hero Killer's quirk is more about punishing those who are 'false heroes'. Izuku would have to struggle with how to use this new power in a way that aligns with his values. Maybe he'd try to reform the quirk's nature, using it to expose real villains who are hiding among the heroes instead of attacking heroes directly.
Aizawa's character might develop in a more complex way. For instance, if the quirk makes him experience something traumatic, he could become more protective of his students. He might also start to question his own abilities and role as a hero. Maybe he realizes that he needs to rely more on others and not just his own quirk. This could lead to him forming stronger bonds with his fellow heroes and being more open to teamwork.