Maybe there are dark mages in this western world. They oppose Naruto because his presence and his different kind of power disrupt the balance of magic they have been maintaining for their own benefits. They would try to use their dark spells to stop Naruto from uniting the west.
Another possible antagonist could be a group of outlaws who have been causing chaos in the west for a long time. They don't like Naruto's attempts to bring law and order as it would put an end to their lawless ways. They might team up with other villains or use underhanded tactics like sabotage to try and undermine Naruto's leadership.
It could be a powerful warlord who sees Naruto as a threat to his control over the western lands. This warlord might have a large army and be used to ruling with an iron fist.
One possible main antagonist could be Orochimaru. Since the Sound 4 were originally his followers, he may not be happy with Naruto leading them. He might try to take them back or undermine Naruto's leadership in various ways.
It could be Orochimaru. Since he had a connection to the Sound 5 in the original Naruto series, he might not be happy with Naruto leading them and try to undermine his leadership.
The Sound Ninjas could be great antagonists. They could use their own unique parkour - like skills to try and stop Naruto. They might be chasing him through the forest, and Naruto has to use his parkour skills to outmaneuver them.
Maybe some corrupt officials within the prison system itself. They oppose Naruto's more humane and reform - focused approach to prison management. They might try to undermine him at every turn, for example, by cutting off resources or spreading false rumors about him among the prisoners.
Maybe a group of dark - type Pokémon that have banded together and are trying to overthrow Arceus. They view Naruto as an easy target at first, but quickly realize he is a formidable opponent with his ninja skills and new Pokémon - related powers. They would constantly try to undermine Naruto and his efforts to protect the Pokémon world.
The Hidden Leaf Village elders could be main antagonists. If Naruto turns dark because he feels betrayed by the village, they would be the ones representing the establishment he is against. Their conservative and sometimes self - serving ways could be the catalyst for Naruto's transformation, and then they would be the ones trying to stop him as he becomes a threat to their power and the village's traditional order.
It could be the traditional enemies from Naruto's world, like the Akatsuki. They might see the union of Naruto and Rias and their unborn child as a threat to their plans for world domination. They could try to kidnap Rias to use her as a bargaining chip against Naruto or to study the unique nature of their child.
The village elders could be antagonists. They might see Naruto as a threat due to his devil - like origin and try to eliminate him. They fear that his powers could bring chaos to the village.
One likely main antagonist could be a powerful demon lord who is trying to break through the barriers between the spirit world and the human world. This demon lord would be a major threat to Naruto and his harem, and they would have to use all their combined skills to stop it.
The main antagonists could be the traditionalist nobles who oppose Naruto's new and inclusive ways of ruling. They might be afraid of losing their power and privilege.