Another possibility is that the main antagonist is a group of villagers who still hold grudges against Naruto for being the host of the Nine - Tails. They oppose his return and will do whatever it takes to stop it, even if it means going against the Hokage's orders. They might be secretly plotting to drive Naruto away again or cause harm to him once he's back in Konoha.
Maybe it's an old enemy of Naruto who has managed to infiltrate Konoha's decision - making process regarding Naruto's return. This enemy could be manipulating the situation to get Naruto in a vulnerable position so they can finally defeat him.
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 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 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 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.
The antagonist could be a new, self - proclaimed ninja god who wants to rule over all the villages. He sees Naruto as the only threat to his power, so he used a powerful spell to take away Naruto's godlike abilities. Konoha wants Naruto back to stop this tyrant. This antagonist might have an army of brainwashed ninja at his command.
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 Orochimaru. Since Orochimaru has always been interested in power and immortality, if Naruto goes back in time, Orochimaru might sense the change in the timeline and try to stop Naruto from changing things that could affect his future plans for power.