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 someone from Naruto's past that he had forgotten about. Maybe a former rival from his academy days who felt overshadowed by Naruto and now that Naruto remembers everything, including some secrets about this person, they become the antagonist. This person might try to stop Naruto from using his new - found knowledge to gain more power or influence in the village.
Perhaps in this fanfiction, Naruto remembers a past life where he was part of a different ninja clan. This could lead to him having skills or knowledge that no one in the current Naruto universe has. For example, he might remember a unique jutsu from his past life and try to recreate it in his present life. He could also struggle with the identity crisis of feeling like he belongs to two different worlds or identities.
It could be a time - traveling enemy from the future who wants to stop Naruto from changing things. This antagonist might have his own motives for wanting the future to remain as it was, like having power over a ruined Konoha.
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 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 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.
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 be a rogue ninja from a neighboring village who is trying to steal some of Konoha's secrets. This ninja might target Konohamaru team because they are young and seem like an easy target, but they underestimate their skills and the protection they have from Naruto.