It could be a powerful devil who sees Naruto as a threat to his rule in the devil realm. Maybe this devil wants to use Naruto's power for his own evil purposes and Naruto has to resist.
Perhaps it's a group of ninja who have always been against Naruto and now that he's a devil, they see it as an excuse to get rid of him. They might fear his new powers and what he could do with them, so they plot against him.
In this fanfiction, the main antagonist could be an ancient evil force that was sealed away long ago but has now been awakened by Naruto's transformation into a devil. This force wants to use Naruto's new form to break free from all the seals and bring chaos to both the ninja world and the devil's domain.
It could be a powerful devil who has the ability to control minds. This devil could manipulate some of Naruto's friends and turn them against him, creating a lot of internal conflict within Konoha.
It could be a rogue ninja who has mastered the devil breakers in a very dark way. He might use them to gain power and control over the ninja world, seeing Naruto as the main obstacle to his plans.
Perhaps an old, powerful shinobi who was thought to be dead but has now returned in Naruto's new life. This shinobi could be seeking to reshape the ninja world according to his own dark vision, and Naruto would have to stop him. The antagonist might have some connection to Naruto's past life that is gradually revealed as the story progresses.
Maybe a new, unknown character. This character could be from a long - lost clan that has a connection to the Book of Life. They believe that the book should be used in a way that Naruto and the other ninjas don't agree with, like using it to rule over all the ninja villages.
The main antagonist could be a corrupted version of Naruto himself from an alternate timeline. This 'evil Naruto' has different ideals and is bent on ruling the ninja world with an iron fist. In Naruto's second life, he has to not only face this powerful version of himself but also figure out how to stop his own dark tendencies from emerging.
It could be a powerful demon lord who wants to conquer both the Naruto universe and the Devil May Cry world. This demon lord might have the ability to manipulate chakra and also use demonic powers, making it a formidable foe for both Naruto and Dante.
It could be Orochimaru. He is always looking for ways to gain power and in a game - like scenario, he could be the main villain trying to stop Naruto from achieving his goals. He might set up various game - like traps and challenges for Naruto.
Perhaps a powerful figure in the Naruto world who senses Naruto's newfound memories and abilities from his past life and sees him as a threat. This could be a rogue ninja or even a hidden power within the ninja villages. They might try to stop Naruto from fully exploring and using his past - life powers.
It could be a new character created specifically for the fanfiction. Maybe a powerful being from a different dimension who sees Naruto's new perfect life form as a threat to his own plans of domination.
It could also be a human scientist who has been experimenting with demonic energy. He becomes corrupted by this power and decides that he should control both the human and demon worlds. He sees Devilman and Devil Lady as obstacles in his path and uses his scientific knowledge and newfound demonic powers to try and defeat them.