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.
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.
Maybe a new character could be introduced as the main antagonist. This character could be from a hidden village that has been secretly observing Naruto's growth. They realize his potential as a weapon and plan to steal his power or use him to start a war against other villages.
Krampus himself could be the main antagonist. Since he is a figure known for punishing the naughty, his actions in the Naruto world might be seen as unjust. He could be going after ninjas without really understanding their motives, causing a lot of trouble for the main characters.
It could be a rogue ninja who also has somehow acquired bankai - like powers. This antagonist might be jealous of Naruto's newfound abilities and wants to prove that he is the stronger one. So he starts causing chaos in the ninja world, challenging Naruto at every turn.
It could be a rogue ninja who wants to disrupt Naruto's business to gain power. Maybe he's jealous of Naruto's success and popularity in the business world. He could try to steal Naruto's business secrets or sabotage his operations.
It could be a rogue Kumo ninja who is jealous of Naruto's power and wants to prove himself by defeating Naruto. This ninja might have some dark secrets and a grudge against Kumo as well.
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.
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 a rogue ninja from a rival village. This ninja has a personal vendetta against Konoha and specifically targets Team 10 because of Naruto's presence. He might be jealous of Naruto's growing power and fame and wants to prove himself by defeating the team.