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.
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.
It could be his immediate superior. If his superior was jealous of Lloyd's intelligence or was under pressure from higher - ups to show some results, he might have used a minor mistake of Lloyd's as an excuse to fire him.
Well, in a fanfiction, SpongeBob could be fired due to a misunderstanding. For example, Mr. Krabs misinterprets something SpongeBob says as a threat to quit. SpongeBob is devastated. He then tries different jobs like being a delivery boy for a seaweed company. However, he misses the Krusty Krab so much. In the end, Mr. Krabs figures out the truth and welcomes SpongeBob back with open arms.
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.
Well, it could be that Harry discovers some major misdeeds by Fudge. Maybe Fudge has been covering up dark magic activities within the Ministry. Harry then finds evidence and presents it to the proper authorities, leading to Fudge's dismissal.
In this kind of fanfiction, perhaps Harry discovers some serious misdeeds by Snape that lead to his dismissal. For example, Snape might be caught unfairly favoring Slytherin students in a major way, like giving them unearned high marks in exams while unduly punishing Gryffindors. Harry, being the observant and just character he is, would report this to the proper authorities at Hogwarts, which could be Dumbledore or the school board. And this could start the process of Snape getting fired.
Well, it could be that Lloyd made a major mistake in his strategic planning for the Geass-related operations. Maybe he miscalculated something crucial about the enemy's movements or capabilities, which led to a significant setback for his side. And as a result, his superiors decided to fire him.
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.