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.
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.
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.
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.
Celty might also interact with him. She's often involved in the events of the series. A drunk Izaya could be a source of confusion for her, or perhaps she would try to help him in some way, seeing as she has a kind heart despite her rather spooky appearance. Another possible character is Namie. Since she has a complex relationship with Izaya, she might be the one to find him drunk and have to deal with the consequences, whether it's covering for him or using his drunken state to her advantage.
Draco Malfoy would be a likely character. Since they are in the same house now, they would have more opportunities to interact. Their relationship could start off as hostile as always, but then gradually change as Hermione shows different aspects of herself in Slytherin.
He might lose his own quirk temporarily. In some fanfictions, a quirk hit can disrupt a character's own power system.
Astrid is likely to be one of the main characters to interact with Hiccup. She might be initially very confused or even a bit jealous depending on how she perceives Hiccup's new form. Stoick, Hiccup's father, would also have significant interactions. He would probably be shocked at first but then try to support his child in this strange situation. And of course, Toothless. Their bond is so strong that Toothless would surely be involved, perhaps trying to understand what has happened to his rider.
Sasuke would definitely be one of the main characters to interact with Naruto. Their relationship has always been complex, and with Naruto as a girl, it could become even more so. Sakura would also interact a lot. She might be both jealous and intrigued by Naruto's new form. Kakashi too, as Naruto's mentor, would have to adjust to this change and guide 'her' in a different way perhaps.
There could also be interactions with the new generation of ninjas. They might view Obito as a sort of legend or a cautionary tale. Their fresh perspectives could either inspire Obito to change further or make him feel even more out of place in this new post - war ninja world.
The two of them could also interact with the Ministry of Magic officials. Harry, being a hero of the war, would have a certain influence. He might try to push for reforms in the Ministry, and Daphne could support him. They would have to deal with bureaucrats who may not be as eager for change, like some of the old - guard pure - bloods who still hold positions of power in the Ministry.