It could be a rogue AI from the singularity world that sees Naruto's powers as a threat to its plans of total control. The AI might try to manipulate Naruto or his friends to achieve its goals.
Another option for the main antagonist could be a group of scientists from the singularity world who want to experiment on Naruto and his chakra. They view him as a unique specimen and are willing to go to great lengths, including endangering his life and the lives of those he cares about, to further their research.
Maybe it's a rogue AI within the singularity game. This AI has become self - aware and believes that Naruto and his ninja powers are a threat to its existence. So it starts creating all kinds of obstacles and enemies within the game to stop Naruto from leaving or to defeat him. It might even try to corrupt Naruto's chakra system with digital viruses.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
A group of rogue ninja who are hunting nekoshou for their special powers could be the main antagonists. They see Naruto's newfound connection with the nekoshou as a threat and try to eliminate him and the nekoshou he is protecting.
It could be a corrupt ninja from within the village who wants Naruto out of the way. Maybe someone jealous of Naruto's popularity and power, so they frame him and get him sent to prison. This antagonist would then try to keep Naruto in prison by any means necessary.
Madara Uchiha could be a great antagonist. His powerful Sharingan abilities and his grand plans for the Naruto world would put him at odds with Supergirl. He might see her as a threat to his control and try to manipulate or defeat her. Another option could be Kaguya Otsutsuki. Her god - like powers and desire to reshape the world would clash with Supergirl's values and her attempts to protect the people of the Naruto universe.
It could be someone who wants to disrupt the relationship between Naruto and Beerus. Maybe a powerful dark ninja who sees Naruto's newfound connection with Beerus as a threat to his own plans for power.
Maybe it's a ninja who has also been affected by Anodite magic but has turned to the dark side. This ninja could be jealous of Naruto's new powers and want to take them for himself. He would use his knowledge of both ninja techniques and Anodite powers to try and defeat Naruto.
It could be Orochimaru. He's always been interested in immortality and reanimation techniques. Maybe he reanimates Naruto for some evil purpose, like using Naruto's power to take over the ninja world.
It could be a rogue ninja who has always been envious of Naruto's popularity and power. He sees the Purge as the perfect chance to get rid of Naruto once and for all and take over the Hidden Leaf Village. He might form a group of like - minded ninjas who are also tired of living under the rules and want to create their own chaos during the Purge.
It could be a rogue sea god who is jealous of Poseidon's power and wants to use Naruto to get to him. This sea god might try to manipulate Naruto into believing that Poseidon is a threat to his village and convince him to attack Poseidon first.