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.
Another potential antagonist could be an inner demon within Naruto that is a manifestation of the Kyuubi's corruption. This inner demon could constantly be battling Naruto's good side, trying to take over completely. And there could be characters who misunderstand Naruto's situation and side with the forces that are trying to 'contain' or 'destroy' Naruto because they believe he is a lost cause due to the Kyuubi's corruption.
In this kind of fanfiction, Gaara could start as an antagonist. He's the new kid from another school who transfers in. He has a very cold and intimidating demeanor. He gets into fights with Naruto and his friends because he has his own ideas about how things should be in the school. He doesn't trust easily and thinks everyone is out to get him. But as the story progresses, he could turn into an ally. Also, Danzo could be an antagonist. He's a strict, old - fashioned teacher who doesn't like Naruto's unruly behavior and tries to expel him from school at every chance he gets.
In the Naruto world, it could be Orochimaru. He might be interested in Naruto's newfound Pokémon - related powers and try to capture him to experiment on him. Orochimaru is always after power, and Naruto's unique abilities could be very appealing to him.
A group of corrupt politicians within the village itself could be the main antagonists. They might be secretly plotting to undermine Naruto's authority as the 5th Hokage in order to gain more power for themselves. They could be making deals with other villages or ninja groups that are not in the best interest of Konoha, and Naruto has to expose and stop their schemes.