Perhaps an old enemy from Naruto's past, who has somehow been resurrected with a new power and a new purpose. This enemy could be someone like Orochimaru, but with even more sinister motives and abilities. He might want to take over the world in a different way than before.
Another option is a new character, a self - made shinobi who believes that Naruto's way of bringing peace is wrong. He thinks that the strong should rule over the weak and has a new purpose of overthrowing Naruto's regime and establishing his own rule of the shinobi world.
It could be someone within the ninja academy who is threatened by Naruto's act of being the dead last on purpose. Maybe a teacher who doesn't like Naruto's deception and wants to expose him.
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.
It could be an alternate version of Orochimaru. Maybe in this new dimension, he has achieved ultimate power and is using it to control the different ninja villages.
It could be an old rival who has also gained some sort of dark god - like powers. For example, Sasuke could have been corrupted by a powerful dark force while seeking more power, and now he stands in Naruto's way as he tries to use his new god - like abilities for good.
It could be a rogue ninja who is jealous of Naruto's new look. This ninja might see Naruto's new appearance as a sign of great power and wants to take it for himself. So, he would constantly try to attack Naruto and steal his power, which would lead to a series of battles between them.
It could be a rogue ninja who was once a part of Konoha but was banished for unethical experiments. Now, with a thirst for revenge against Naruto and the village, he returns as a powerful and unpredictable foe.
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.
A powerful rogue ninja who has mastered a forbidden jutsu that gives him the ability to control minds could be a great main antagonist. He could use this power to turn Naruto's friends against him, creating a lot of internal conflict within the group.
The Joker could be a great antagonist. His chaotic and unpredictable nature would contrast with Naruto's more straightforward sense of justice.
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.