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.
Perhaps a corrupted version of Naruto himself from this new dimension. He could have been influenced by dark forces and turned against the values he once held. This would create an interesting conflict as the real Naruto has to face this corrupted version of himself and try to bring him back to the light while also dealing with the consequences of his actions in this new dimension.
A group of rogue ninja from the other dimension could also serve as the main antagonists. They might be jealous of Naruto's power from his original dimension and want to steal it for themselves. They could be led by a charismatic and devious leader who has a personal vendetta against Naruto for some reason, like he believes Naruto will disrupt their way of life in the dimension.
A powerful dark wizard could be the main antagonist. This wizard has been ruling the other dimension with an iron fist and sees Peter as a threat to his power. He uses his dark magic to create all kinds of obstacles for Peter, from summoning monsters to casting spells that distort Peter's perception of reality.
A powerful and corrupted duelist from a forgotten dimension could be the main antagonist. This duelist might have discovered a way to harness the power of the dimensional rifts for his own evil purposes. He could be using this power to create chaos and destruction across all the dimensions, and only the combined efforts of the heroes from different dimensions can stop 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 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 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.
Perhaps a group of rogue time - traveling wizards. They come from a future in this dimension where the wizarding world has been destroyed, and they believe that changing the past (even if it means causing chaos) is the only way to save their future. They see Harry as a threat to their plans because he has a natural inclination to preserve the present order.
Maybe it's a corrupted version of one of the gods from Percy's world who has gained even more power in this new dimension. They could be trying to spread chaos and destruction, and Percy has to stop them. This antagonist would know Percy's weaknesses from their time in the original world, which would make the conflict more intense.