It could be that the very concept of the hellish order itself is the antagonist. If Naruto wants to bring a more positive change, the established rules and the entities that enforce them would be against him. For instance, the guardians of the different levels of hell might not want Naruto to interfere with their way of doing things and would fight to keep the status quo.
There could be a powerful overlord in Hell who rules the place. Naruto's presence and his potential to grow strong might be seen as a threat to this overlord's power. So, this overlord would be one of the main antagonists, constantly sending minions or directly confronting Naruto to keep him in check.
One possible main antagonist could be a rogue ninja from a forgotten clan who has a vendetta against the Land of Fire. They might be seeking revenge for something that happened in the past and see Naruto as the symbol of the new regime they want to topple.
Some of the more powerful demons in Hell itself could oppose Percy. They might be jealous of his sudden rise to power as the Prince of Hell. These demons could have their own agendas and territories in Hell that they don't want Percy interfering with. They could use their dark magic and legions of lesser demons to try and get rid of him.
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.
Perhaps the story is centered around Naruto becoming the 'Lord of Hell'. He might use his newfound power and status to bring about changes in the hellish environment. He could try to reform the system there, like making it more just for the souls trapped, all while using his ninja skills in unexpected ways. For example, using shadow clones to manage different tasks in the vast expanse of hell.
The Death Eaters could still be the main antagonists. Even if Harry is Lord Peverell, they would oppose him as he is still a threat to their dark ideology.
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.