It could be a mysterious inmate who has his own agenda. He might be using the chaos in the Blood Prison to his advantage. For instance, he could be secretly gathering power from the negative emotions of the other inmates and planning to break out and cause havoc in the ninja world. Naruto has to figure out his plan and stop him before it's too late. This inmate could have some connection to an old enemy of Konoha that Naruto doesn't know about at first.
Perhaps a corrupt warden who is using the Blood Prison for illegal activities like smuggling forbidden jutsu scrolls or using the inmates as test subjects for new, dangerous jutsu. He sees Naruto as a threat to his operations and tries to keep him locked up and quiet.
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 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.
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.
Perhaps an old member of a forgotten clan who was exiled because of his dark blood. He returns with a vendetta against the village that shunned him. He sees Naruto as a symbol of everything he hates and tries to destroy him. The main antagonist could also manipulate other ninja to turn against Naruto by spreading fear about his dark blood.
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 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.
The Organization in the Claymore world could be the main antagonist. They already have a lot of secrets and do some morally ambiguous things. If Naruto shows up and starts changing the status quo, like teaching Claymore warriors to be more independent, the Organization might see him as a threat and try to eliminate him.