A possible main antagonist could be an outside force, like a member of a rival ninja village. They infiltrate the Hidden Leaf Village, frame Naruto, and ensure he's sent to prison. Their goal might be to weaken the Leaf Village by removing one of its strongest assets. They'd then try to cause chaos within the prison to prevent Naruto from ever getting out.
It could be a corrupt ninja from within the village who wants Naruto out of the way. Maybe someone jealous of Naruto's popularity and power, so they frame him and get him sent to prison. This antagonist would then try to keep Naruto in prison by any means necessary.
It could be the warden of the Blood Prison. He might be a corrupt figure who is using the prison for his own gain. He sees Naruto as a threat to his operations and tries to keep him locked up at all costs.
It could be one of the powerful and corrupt prison guards. They might have their own hidden agendas and see Naruto as a threat to their power or secrets within the Blood Prison. So, they start a scheme to make it seem like Naruto has betrayed everyone.
The warden of the Blood Prison could be the main antagonist. He might be a power - hungry and corrupt individual who sees Naruto as a threat to his control over the prison. So, he devises plans to get rid of Naruto, which ultimately lead to Naruto's death.
Might be one of the powerful guards who has a grudge against Naruto for some reason. Maybe he believes Naruto is a threat to the order in the prison and tries to get rid of him secretly.
The main antagonist could be a rogue ninja who has infiltrated the village's council. This ninja could be secretly manipulating things behind the scenes to get Naruto sent to prison. He might have a vendetta against Naruto for some past encounter or just see him as a threat to his own evil plans.
Maybe Danzo could be the main antagonist. He has always been a bit of a shady character in the Naruto series. He could see Naruto as a threat and use his influence to frame him and send him to prison to further his own political or power - hungry goals.
It could be a rogue ninja who has infiltrated the village's council and framed Naruto. This ninja might have a vendetta against Naruto or the village and sees this as an opportunity to create chaos.
It could be a corrupt prison warden who has a personal vendetta against Harry. This warden might make Harry's life in prison extremely difficult and try to prevent him from ever getting out, even if he's innocent.
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.
Perhaps the main antagonist is a corrupt official within the Hidden Leaf Village itself. This person was the one who arranged for Naruto's imprisonment in the first place and now that Naruto is out, they will do everything to stop him from uncovering their misdeeds. They might even team up with external enemies to achieve their goals.