Anko might not be a traditional antagonist, but if she is siding with Evil Naruto for some reason, she could be seen as an antagonist from the perspective of the other villagers. However, it's also possible that she is being manipulated herself, in which case the true antagonists would be those doing the manipulating.
The main antagonist could be the evil Naruto himself. Since he has turned to the dark side, his actions and motives would be against the well - being of the village and those around him, including Kushina. He might be the source of conflict and chaos.
Perhaps a rogue ninja group that has a vendetta against Anko. When she forms this new family with Naruto, they target the whole family unit to get back at her. They might see Naruto as a way to get to Anko or vice versa, and constantly attack and create chaos for them.
The Hidden Leaf Village itself could become the main antagonist. Since Naruto turns evil, the village that once nurtured him would now be against him. The village elders and some of the ninja who oppose Naruto's new evil ways would be in conflict with him. For example, Kakashi might be forced to fight against Naruto to protect the village.
The Hidden Leaf Village itself could be an antagonist. If Naruto goes evil, the village that once was his home would be against him now.
There could also be new characters introduced as antagonists. In this fanfiction, perhaps there are dark entities that are either drawn to Naruto's new evil self or are competing with him for control. They might have powers that rival or complement his Rinnegan - based abilities, leading to some intense battles and power struggles.
Maybe a new group of rogue ninjas who see Naruto as a threat to their plans. They've kidnapped him and are holding him captive. Anko would then have to fight these ninjas who are well - trained and have their own unique abilities. They might be after Naruto's Nine - Tails chakra for some evil purpose.
The villagers themselves could be the main antagonists. Since Naruto has been neglected, their continued mistrust and hostility towards him could drive his 'evil' turn. They represent the status quo that has failed Naruto, and he might see them as the source of his problems.
Sasuke could also become an antagonist in this scenario. If Naruto turns evil, Sasuke, who has his own sense of justice, would likely oppose Naruto. Their once - strong friendship would be shattered, and they would end up fighting each other, with Sasuke trying to stop Naruto from causing more destruction.
The Akatsuki as a whole could be antagonists. In fanfictions, the Akatsuki might be interested in the power of Naruto's Evil Rinnegan. They could try to capture him to extract the power or use him in some way for their own goals. Pain, being a powerful member of the Akatsuki, could also be a specific antagonist. His own ideology about pain and peace might clash with Naruto's new, potentially more chaotic state with the Evil Rinnegan. Additionally, new original antagonists created specifically for the fanfiction could be common. These could be dark, powerful figures who are drawn to the power of the Evil Rinnegan and want to either possess it or destroy Naruto to prevent its misuse.
Well, the main antagonists could be a group of rogue ninja who are hired by a powerful nation to take down Naruto. Since he is unkillable, they would have to use more underhanded tactics like trying to isolate him from his friends and allies. Or perhaps there is a dark force that is awakened by Naruto's unkillable state, thinking that he is a threat to the balance of the ninja world. This dark force could then become the main antagonist, trying to destroy Naruto at all costs.