Perhaps in the fanfiction, Aizawa gets One For All and decides to keep it a secret. He then starts training in isolation to master it. As he trains, he discovers new aspects of the power that are different from what previous users experienced, due to his own unique quirk and personality. He might use it to secretly protect his students from threats that are beyond the scope of his normal abilities.
One possible plot could be that due to some false accusations of improper teaching methods, Aizawa gets fired. His students then band together to prove his innocence. They collect evidence of his effective teaching, like how he has helped them improve their quirks and grow as heroes in training. Eventually, they manage to clear his name and he is reinstated.
One possible plot could be Naruto suddenly obtaining the One for All power. He might struggle at first to control it as it's a very different kind of power from the chakra he's used to. His friends would be both amazed and a bit worried. Naruto could then try to incorporate this new power into his fighting style, perhaps creating new jutsu that combine his ninja skills with the raw strength of One for All.
He would likely become more confident in his abilities. Aizawa is already a capable hero, but One For All would give him an extra edge. This new power might make him a bit more assertive in hero situations, rather than just being the stealthy and observant hero he was before.
It could be a new administrator at the school who has a very different vision of how hero training should be conducted. This person might see Aizawa's methods as too old - fashioned or too lax, even though they are actually effective. So, they push for his dismissal.
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, it could be about Izuku confronting Aizawa over some misrepresentation in the fanfiction. Maybe Izuku doesn't like how Aizawa is being portrayed as too strict or unfair in the fic.
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.
One possible plot could be that Izuku, under All for One's influence, has a completely different view of hero society. He might be trained in unique and perhaps unethical ways, leading to a conflict within himself as he tries to figure out his own identity and what it means to be a 'hero' or a 'villain'.
Maybe Kakashi could still be his mentor. Even though One for All is a new and different power, Kakashi is very experienced. He could help Naruto understand how to incorporate this new power into his overall combat strategy. Kakashi would be able to analyze the power and give Naruto advice on how to train with it while still relying on his ninja skills.
Maybe Joho's sickness could be a magical ailment that only Saphie, the one - eyed cat, can cure. For example, Saphie might have some special powers that are connected to her one eye, like the ability to see into the spirit world where the source of Joho's illness lies.