It could be an old enemy who has resurfaced and found a way to turn Naruto against the Leaf. Maybe Orochimaru, who has always been interested in Naruto's power, manipulates him into attacking the village. Orochimaru could use his knowledge of dark jutsu to influence Naruto's mind.
Perhaps it's a new character introduced in the fanfiction. This character might have a vendetta against the Leaf for some past wrong and sees Naruto as a powerful tool to exact revenge. They could use false information and emotional manipulation to make Naruto believe that the Leaf is his enemy.
The antagonist could also be a group. For instance, a group of rogue ninjas who want to see the downfall of the Leaf. They could kidnap Naruto's loved ones and force him to attack the Leaf under threat. This would create a complex situation where Naruto has to balance protecting his loved ones and not causing harm to the village he once called home.
In this fanfiction, perhaps Naruto could be under some sort of mind control. Maybe an enemy has found a way to manipulate him into attacking the Leaf. It could also be a case where Naruto is misled about the true nature of the Leaf Village, believing it has committed some great wrong that warrants his attack.
It could be Orochimaru. He has always had his eyes on Konoha and might find a way to manipulate Naruto into attacking it. Maybe he uses some new experimental jutsu on Naruto to make him do his bidding.
It could be a rogue ninja from another village who wants to use the demon within Naruto for his own evil purposes. He might be power - hungry and sees Naruto as a tool to gain control over the Hidden Leaf and other ninja villages.
It could be a rogue ninja from the Hidden Leaf's past who has come back for revenge. Maybe someone who was wronged by the village long ago and has now amassed great power to destroy it. Another possibility is a new enemy from outside the village, like a powerful warlord from a far - off land who wants to conquer the Hidden Leaf for its resources.
It could be a new and powerful trainer who has a vendetta against Ash. This trainer might see Ash as a threat to their own rise to the top in the Pokemon world. They could manipulate Leaf into betraying Ash to further their own goals.
Sasuke could be one to stop Naruto. They have a complex relationship and Sasuke might understand Naruto's motives and be able to talk him down.
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.
Krampus himself could be the main antagonist. Since he is a figure known for punishing the naughty, his actions in the Naruto world might be seen as unjust. He could be going after ninjas without really understanding their motives, causing a lot of trouble for the main characters.
It could be a rogue ninja who also has somehow acquired bankai - like powers. This antagonist might be jealous of Naruto's newfound abilities and wants to prove that he is the stronger one. So he starts causing chaos in the ninja world, challenging Naruto at every turn.
It could be a rogue ninja who wants to disrupt Naruto's business to gain power. Maybe he's jealous of Naruto's success and popularity in the business world. He could try to steal Naruto's business secrets or sabotage his operations.