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.
Maybe a powerful and vengeful inmate who has a grudge against the Leaf Village. He sees Naruto as a symbol of the Leaf and wants to take revenge on him. So he tries to make Naruto's life in the prison a living hell and also plots to kill him.
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.
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.
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.
Well, one possibility could be that Naruto is framed for a crime he didn't commit and sent to the Blood Prison. There, he might face all kinds of dangerous inmates and tough prison guards. He would probably try to prove his innocence while also having to deal with the harsh prison environment and maybe some secret plots within the prison walls.
It could be the Devil or some powerful demon lord in hell. This antagonist would try to keep Naruto trapped in hell and prevent him from escaping. He might see Naruto as a threat to his rule in hell or just enjoy tormenting him.
One possible main antagonist could be the ruler of hell. This powerful being would see Naruto as an intruder and a threat to the order of hell. Another could be a demon who has a vendetta against Naruto for some reason, perhaps because Naruto's positive energy is anathema to the demon's evil nature.
It could be a rogue ninja who has mastered the dark side of Blood Blockade Battlefront techniques. He might be seeking power to take over the ninja world and uses his knowledge of this unique technology to create chaos.
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.