There's a possibility that the main antagonist is a group of rogue ninjas who are using the chaos of the monster - filled world to their advantage. They are capturing and selling the monsters for some evil purposes, and Naruto has to stop them as they are causing more harm to the already troubled world.
It could be a powerful ninja who has the ability to control the monsters. He might be using the monsters to achieve his own goals, like taking over the Hidden Leaf Village.
It could be a new villain who has risen to power during Naruto's absence. This villain might have a vendetta against Konoha or the ninja world in general because of something that happened while Naruto was away.
It could be a rogue ninja who has made a pact with the demons in the demon world. This ninja has turned against the ninja world and wants to use the power of the demon world to rule over all. He would constantly oppose Naruto's attempts to restore peace.
Perhaps a group of elite warriors in the new world who are jealous of Naruto's unique abilities. They form an alliance to take him down because they fear he will disrupt their status quo and take over their positions of power.
It could be a powerful kunoichi who opposes Naruto's presence in her world. Maybe she sees him as a threat to the existing power balance among kunoichi.
It could be an ancient sorcerer who has mastered the Monsters and Mana magic and wants to use Voltron's power for his own evil deeds. He might try to control the Voltron lions using his mana spells.
It could be a powerful wizard in the new world who sees Naruto as a threat to his rule. Since Naruto has unique powers from his own world, the wizard might want to eliminate him or use his powers for his own evil purposes.
Well, it could be about Naruto finding himself in a world where the normal animals are replaced by all kinds of monsters. Maybe he has to learn new jutsus to fight them or find a way to co - exist with them.
Obviously, the Primordial of Monsters itself could be the main antagonist. It has the power and the motive to cause chaos and destruction, which would put it directly in opposition to Percy and his friends.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
Maybe a new character could be introduced as the main antagonist. This character could be from a hidden village that has been secretly observing Naruto's growth. They realize his potential as a weapon and plan to steal his power or use him to start a war against other villages.