Another possibility is a new, original antagonist created specifically for this fanfiction. This character could be jealous of Naruto's early success and want to bring him down. They might be from a rival village or a secret organization within Konoha that doesn't like the idea of a 5 - year - old being so advanced in their ninja training.
It could be Orochimaru. Since Naruto is graduating early and showing great potential, Orochimaru might see him as an interesting target for his experiments or as a threat to his future plans earlier than in the original story.
Maybe Danzo. He is always one to be wary of those with great power and potential. Naruto graduating at 5 with obvious talent might make Danzo see him as a loose cannon that could disrupt his vision for the village. So, he could try to manipulate or even eliminate Naruto.
It could be Orochimaru. Even though Naruto graduates early, Orochimaru might see him as a threat and try to eliminate him. Orochimaru is always on the lookout for powerful ninja to either recruit or destroy.
In this fanfiction, Naruto might be shown as an even more precocious and determined character. Since he graduates at 5, he could have shown extraordinary skills and intelligence from a very young age. Maybe he has already mastered some basic jutsus better than his peers, which could lead to some jealousy among them.
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.
One possibility is that Naruto has a secret training regime that he has been following. This extra training gives him an edge over his classmates. He could then participate in a tournament within the academy where he completely outperforms everyone. His display of strength, speed, and strategic thinking would be so impressive that the academy decides to graduate him early.
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.