The main antagonist could be a new character created specifically for the fanfic. This character could be a powerful ninja from a far - off land who has a grudge against Konoha and uses Naruto as a pawn in his plan to destroy the village. He might have some sort of power that allows him to control Naruto's actions.
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 a neighboring village who has a vendetta against Konoha. This ninja might see Naruto's return as an opportunity to take on the strongest ninja in Konoha and prove his own strength.
The Konoha elders could be the main antagonists. They are often shown to be quite traditional and set in their ways in the original series. If Naruto goes against their old - fashioned ideas about the village's future and ninja code, they would likely oppose him strongly and could be the driving force behind making Naruto an outcast, which in turn could lead to his 'betrayal'.
It could be a new villain who has been plotting against Konoha while Naruto was away.
It could be a rogue ninja from another village who has a vendetta against Naruto. This ninja might spread false rumors about Naruto in Konoha, which eventually leads to Naruto leaving. So, this ninja would be the main antagonist as he is the root cause of Naruto's departure.
It could be some of the more conservative village elders in Konoha. They might see Naruto Namikaze's unorthodox ways and growing power as a threat to their traditional way of running the village. So they create situations that make Naruto feel unwelcome and eventually lead to his departure, making them the antagonists in the story.
It could be a rogue ninja from within Konoha who doesn't want Naruto to return because he fears Naruto's growing power will disrupt his own plans for power within the village.
It could be Orochimaru. He's always been interested in Konoha's secrets and might have found a way to use Naruto's clones for his own purposes. He could be manipulating the clones from the shadows to steal forbidden jutsu scrolls or something similar.
Perhaps Danzo could be the main antagonist. He was always a bit of a shadowy figure in the original Naruto series. In this fanfic, he might see Naruto and Sasuke as a threat to his own control over Konoha. So, he uses his influence to have them banished. He could be secretly working with external forces to further his own agenda while keeping Naruto and Sasuke away from Konoha.
It could be a corrupt elder in Konoha. They might be power - hungry and see Naruto as a threat to their control. So they manipulate the situation to make Naruto the enemy.
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.