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 hidden group within Konoha that was behind Naruto's banishment. They might be afraid of his growing power and influence. This group could work in the shadows to stop Naruto from ever returning to Konoha and regaining his status.
One possible plot could be that Naruto and Sasuke are wrongly accused of a serious crime against Konoha. For example, they might be framed for conspiring with an enemy village. As a result, they are banished. Naruto, with his unwavering spirit, might start thinking about how to prove their innocence from outside the village. Sasuke, on the other hand, could initially be more bitter and focused on getting revenge on those who framed them. Together, they embark on a journey to find the real culprits while also facing various challenges outside Konoha, such as dealing with other ninja clans who see them as outcasts and potential threats.
It could be some of the village elders. They were the ones who decided to banish Naruto, so they could continue to oppose him even when he's in exile. They might see his growing power and influence as a threat to their traditional ways of running the village.
The main antagonist could be an old - school ninja who believes in the traditional and often harsh ways of the ANBU. They see Naruto's actions as weakening the village and set out to oppose him at every turn, trying to bring back the old ANBU regime.
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.