The main antagonist could be a new character. Someone who has a vendetta against Naruto's family or the Hidden Leaf Village in general. This character could be secretly plotting against Naruto and the village, and Naruto's journey to get even would be to stop this new threat. They might have some connection to an old shinobi clan that was wronged in the past, and they see Naruto as a symbol of the current system that they want to topple.
It could be Sasuke. I know they are friends in the original, but in this fanfiction, Sasuke might turn against Naruto for some reason. Maybe he becomes jealous of Naruto's newfound power or status.
A new and powerful ninja from a far - off land could be the main antagonist. This ninja has heard of Benihime's power and wants it for himself. He sees Naruto as an easy target at first but soon realizes that Naruto is not going to give up Benihime without a fight. This antagonist could have unique powers that are a challenge for Naruto to overcome while protecting Benihime.
Sasuke could potentially be an antagonist. With Naruto having the Rinnegan, Sasuke might see him as a threat to his own power and goals. This could lead to a new rivalry between them, different from their previous one.
It could be Orochimaru. He has always been interested in power and Naruto's unique abilities. With Naruto sealed away, Orochimaru might see it as a chance to steal whatever power Naruto had or use his absence to further his own evil plans in the ninja world.
It could be someone who is jealous of Naruto getting the monkey contract. Maybe a rogue ninja who wants the power for himself.
It could be a hidden group within Konoha that was behind Naruto's banishment. They might be afraid of his growing power and influence. This group could work in the shadows to stop Naruto from ever returning to Konoha and regaining his status.
It could be Orochimaru. He's always interested in powerful artifacts, and the mask Naruto gets would be no exception. Orochimaru would likely try to steal the mask from Naruto to add to his collection of powerful items and use its power for his own dark experiments.
Maybe an old enemy who was thought to be defeated but has somehow returned stronger. Orochimaru could be a possibility. He was always obsessed with power and immortality. If Naruto is revived, Orochimaru might see it as an opportunity to study Naruto's new state of being (since he was revived) and use that knowledge for his own dark purposes.
The main antagonist might be an organization that wants to take over the Hidden Leaf Village. They see Naruto as a major obstacle because of his strength and influence among the villagers. So, they devise a plan to make everyone in the village betray Naruto. They use mind - control techniques or spread misinformation on a large scale to achieve their goal. This way, they can weaken the village from within before making their move to take it over.
Another option is a group of anti - magic fanatics. They view Percy's wings as an abomination because they are a result of magic. They hunt Percy down, believing that by getting rid of him and his wings, they are protecting the world from the 'evils' of magic.
It could still be Hawk Moth. Since Nathalie has been working with him, getting a Miraculous might make her a threat to his plans, so he could turn against her and become the main antagonist.