Maybe the main antagonist is a former student of All Might who turned to the dark side. This person is jealous that Uraraka got One for All instead of them. So they try to take the power from her and cause chaos in the hero world. They would use their knowledge of One for All against Uraraka, making it a difficult battle for her.
One possible plot could be that Uraraka struggles to control the new power of One for All at first. She might accidentally use it in inappropriate situations, like floating everything in her dorm room. But as she trains hard, she gradually masters it and uses it to save her friends in a big battle against a powerful villain.
It could be someone who wants to steal the One for All bloodline from Naruto. Maybe a rogue ninja from another village who has heard about this new and powerful ability in Naruto and decides to make it his own.
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.
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.