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.
Well, it could be that Nero accidentally breaks some important rule in his world. Maybe he accidentally damages a powerful relic that is crucial for his community's safety, and then he has to face the wrath of the elders or those in charge.
Another option is a group of anti - magic fanatics. They view Percy's wings as an abomination because they are a result of magic. They hunt Percy down, believing that by getting rid of him and his wings, they are protecting the world from the 'evils' of magic.
It could still be Hawk Moth. Since Nathalie has been working with him, getting a Miraculous might make her a threat to his plans, so he could turn against her and become the main antagonist.
It could be a long - lost relative of an old enemy. For example, if there was a powerful dark wizard that Natsu defeated in the past, this relative might come seeking revenge and be powerful enough to kill Natsu.
Maybe an evil artist from another dimension. This artist is jealous of Nathanael's pure creativity and the unique power of his miraculous. They come to Nathanael's world to steal his ideas and use his miraculous for their own dark purposes, like creating chaos and destruction through art.
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.
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.