It could be a rogue ninja from within Konoha who has embraced the 'darkness' fully and wants to reshape the village according to his own twisted ideals.
Another possibility is a powerful and corrupt shinobi who has risen to power within the 'Darkness Konoha' faction. He sees Naruto as a threat to his power and plans to eliminate him before Naruto can expose the secrets of the 'Darkness Konoha'.
Maybe an old enemy of the founders of Konoha who has been biding his time in the Darkness Konoha area and now decides to strike. He might have a vendetta against the current state of Konoha and wants to destroy it.
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.
Perhaps it's an old enemy who has somehow managed to come back stronger. Maybe someone from Orochimaru's old experiments that went wrong, but instead of dying, they gained some dark and powerful abilities. They see Naruto's return as a threat to their plans of world domination, so they target Konoha first. They might be able to control dark chakra in a way that no one has seen before, and use it to create chaos and destruction. They could also have some minions who are equally as powerful and loyal to them, making them a formidable foe for Naruto and Konoha.
The elders of Konoha could be the main antagonists. They might be the ones who have been neglecting Naruto all along, and when he leaves, they see him as a threat to Konoha's stability.
It could be a long - lost relative of one of the old villains like Orochimaru. Maybe this person has been in hiding, training and growing in power, and now wants to take revenge on Konoha for what happened to their family member.
The main antagonist could be a new character. For example, a powerful ninja from a rival village who has a vendetta against Konoha. This ninja could manipulate Naruto into leaving Konoha by spreading false rumors about the village. Then, he tries to use Naruto's power as an assassin for his own evil plans.
Maybe the main antagonist is a powerful figure within Konoha who doesn't want Naruto to leave and become an independent force. He secretly sends assassins after Naruto. When Temari gets involved, this figure also targets her. His actions are driven by his thirst for power and fear of Naruto's potential outside of Konoha's control.
A new character could be introduced as the main antagonist. Maybe a powerful ninja from another village who has infiltrated Konoha's politics. This character could be the one behind the idea of forcing Naruto back, with the intention of using Naruto as a pawn in a larger scheme against Konoha.
It could be Orochimaru. He has always been interested in power and has no qualms about using dark methods. He might see Naruto as a perfect vessel or a tool to further his own dark plans, and through his manipulations, Naruto starts to be consumed by darkness.
It could be an old ninja who was once banished from Konoha for using dark jutsu and now wants revenge. He might see Naruto as a symbol of the village he hates and tries to corrupt him by luring him into the darkness.
A powerful dark spirit might be the main antagonist. This spirit could sense Naruto's new ability to control darkness and see him as a threat. So it starts causing chaos in the ninja world, forcing Naruto to use his dark - controlling powers to stop it. The spirit could have the ability to possess other ninjas, making them do its bidding.