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.
Madara Uchiha could also be a likely main antagonist. His power and his own dark desires could easily draw Naruto into the abyss. Madara might convince Naruto that the world needs to be remade in a more 'ruthless' way, and Naruto, in his vulnerable state, might start to believe him and thus be consumed by the associated darkness.
The Akatsuki as a whole could be the main antagonists. They are a group associated with various dark deeds. They might target Naruto not just for the Nine - Tails, but also to corrupt him. By constantly attacking him and his loved ones, they could push Naruto to the brink where he loses hope and is consumed by the darkness they represent.
It could be the Hidden Leaf Village itself. If the village turns against Naruto through false accusations or unjust treatment, it becomes the force that drives him to the dark side. The elders or some of the more conservative ninja in the village who oppose Naruto's unorthodox ways could be the face of this antagonism.
It could be an ancient evil that has been lurking in the shadows for ages. Maybe a dark spirit that was sealed away long ago and is now breaking free, and Naruto has to control the darkness within himself to fight it.
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.
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.
A group of power - hungry shinobi from a hidden village could be the main antagonists. They see the 'angel of darkness' as a weapon that they can use to gain supremacy over other villages. They start a series of attacks and schemes to capture the angel, putting Naruto and his allies in a difficult position as they try to protect it and keep the peace in the ninja world.
A powerful sorcerer who wants to control the power of the fairy of darkness for his own evil purposes could be a great antagonist. He might try to trap her and force her to do his bidding.
It could be a powerful dark wizard who wants to use Sakura's powers for his own evil purposes. He might be constantly trying to capture her or manipulate her into doing his bidding.
One possible plot could be Naruto being influenced by a powerful dark entity. Maybe he starts to question his values and the way of the ninja. For example, he might be lured by the promise of power that the darkness offers, like the ability to defeat all his enemies easily.
Another antagonist could be a human with a grudge against Danny. This person discovers Danny's 'Prince of Darkness' potential and decides to use it against him. They might team up with some lesser ghosts and use dark magic or technology to try and take Danny down. They could be motivated by jealousy or a desire for power.