There could be a parent of one of the students who is very influential. This parent doesn't like Aizawa's no - nonsense approach and believes it is too harsh on their child. They use their influence to get him fired. But as the story progresses, they come to realize their mistake when they see how the students are actually thriving under Aizawa's guidance.
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.
Maybe a jealous colleague could be the main antagonist. This colleague might feel overshadowed by Aizawa's popularity among the students. So, they fabricate stories and evidence to get him fired, hoping to take his place or gain more influence in the school.
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.
Maybe the main antagonist is an old enemy of Robin's who becomes jealous of his new powers. This enemy then devises a plan to steal Robin's powers or at least neutralize them so that he can regain his former status as the top threat. He could be a powerful martial artist or a criminal mastermind with a large network of henchmen.
It could be a jealous rival. This rival might be envious of mankae's popularity or achievements. So, they set him up to get arrested by providing false information to the police. They want to see mankae's downfall and think that getting him arrested is the best way to achieve this.
It could be Orochimaru. Since he has always been interested in powerful ninja abilities, he might see Sakura with the Sharingan as a new target. He could try to steal the Sharingan from her to add to his own collection of powerful jutsus and abilities.
A power - hungry politician in Nero's world could be the main antagonist. They see Nero as a threat to their power or as an easy target to gain more influence. They manipulate situations to make it seem like Nero has done something wrong.
Maybe it could be a mad scientist who wants to study the chibi. He sees it as a unique specimen and is willing to do anything to get his hands on it.
It could be a new villain created specifically for the fanfiction. Maybe someone who has a vendetta against Chloe and targets her because she has the Miraculous.
A rogue robotic entity could be the main antagonist. This entity has the ability to roboticize other beings and views Tails as a prime target. It wants to use Tails' body as a vessel to spread its robotic influence further, so it sets out to roboticize him and then use him as a tool for its own dark purposes.
Another possibility is a corrupted version of one of Sonic's friends. Maybe through some dark influence, like an evil Chaos Emerald, a friend turns on Sonic and blinds him. This would add an extra layer of drama as Sonic has to deal not only with his blindness but also the betrayal of someone he trusted.
It could be Sasuke. I know they are friends in the original, but in this fanfiction, Sasuke might turn against Naruto for some reason. Maybe he becomes jealous of Naruto's newfound power or status.