Maybe one of the rogue ninja who has a vendetta against Naruto's village. They could plant evidence to make it look like Naruto is working with them or has committed a crime against another village. Their goal would be to not only get Naruto out of the picture but also to cause chaos in his village. So, this rogue ninja would be the main antagonist as they work to keep Naruto in jail.
It could be Danzo. He's always been a bit of a shady character in the Naruto series. He might see Naruto as a threat to his power and influence in the village. So, he could frame Naruto for some crime and get him sent to jail just to eliminate his competition.
The Joker could be the main antagonist. Even though Batman is in jail, the Joker could find a way to continue his chaos - making. He might try to get into the prison to torment Batman or use the situation to his advantage by causing mayhem in Gotham City without Batman there to stop him.
The Kingpin could be a likely antagonist. He has the resources and the motive to frame Spider Man and get him out of the way. He could use his criminal connections to plant false evidence and make sure Spider Man ends up in jail.
The main antagonist could be a new character, a corrupt official in the Ministry of Magic who wants to get rid of Harry because he's seen as a threat to their power. This person could manipulate the legal system to send Harry to jail.
It could be a new character, perhaps a corrupt wizard in the Ministry of Magic who has a vendetta against the Dursleys for some reason. This wizard might make their time in jail even more difficult by withholding basic rights or subjecting them to unfair treatment. And he might be doing all this for his own gain, like trying to steal the Dursleys' Muggle money or something.
It could be the same people who framed Naruto and got him jailed in the first place. Maybe a group of rogue ninjas or corrupt village leaders who see Naruto as a threat to their power.
The main antagonist could be Naruto himself in a sense. Since he has gone insane, his actions are now unpredictable and harmful to the people around him. His former friends, like Sakura or Kakashi, might have to step up and try to stop him, seeing him as the new threat.
The Hidden Leaf Village itself could be the main antagonist. If they turn on Naruto for no good reason, they become the force he has to fight against as a rogue ninja.
Maybe it could be a new character who is a combination of ninja and wizardry evils. This character could be jealous of Naruto's unique abilities that bridge the gap between two different worlds and wants to stop Naruto from uniting the ninja and wizarding communities against him. They could have powers that are a mix of dark magic and some dark ninja jutsus.
It could be a corrupt ninja from another village who wants to get rid of Naruto as he sees him as a threat to his own plans. This ninja could have framed Naruto to send him to prison.
Well, perhaps Naruto accidentally breaks a really important and ancient law in the ninja world. It could be something related to a forbidden jutsu that he was unknowingly involved in. As a result, he's thrown into jail. There, he might start to reflect on his actions and ninja way, and also try to find a way to prove his innocence while making some unexpected allies among the prisoners.