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 rogue ninja who was once a part of the Akatsuki but managed to escape before their downfall. He's been biding his time, growing stronger, and now he wants to take revenge on Naruto and the Hidden Leaf for what happened to the Akatsuki.
A group of rogue ninjas from other clans who oppose the idea of the Hidden Leaf Village. They see the Shodai Hokage's leadership as a threat to their own power and freedom. They might team up and use underhanded tactics like assassinations, sabotage of the village's resources, and spreading false rumors to try and undermine the Shodai Hokage's authority. They could also form alliances with other enemies of the village, making them a formidable and complex antagonist group.
It could be someone who wants to expose Naruto's secret. Maybe a rogue ninja who wants to use this information to create chaos in the village by making the villagers doubt Naruto's intentions.
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.
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.
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 from another village who has a vendetta against Konoha. Maybe he blames the Godaime Hokage for something that happened in the past, like the death of his family during a Konoha - related mission. So, he decides to target the village and the Godaime Hokage in particular.
Perhaps the main antagonist is a spirit or a curse that has been awakened. This curse could be slowly influencing the villagers to turn against Naruto. It could be something that was sealed away long ago and now has broken free due to some mistake or interference. The antagonist could also be a mastermind from another hidden village who is using spies and psychological warfare to undermine Naruto's rule as the Sixth Hokage.