It could still be Orochimaru. His plans for immortality and power - seeking don't really depend on Naruto's birth time. So he would still be a major threat to the village and Naruto himself.
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.
Maybe there is an old, powerful being who sees Percy's wings as a threat to his rule. This antagonist would try to stop Percy from fully realizing the potential of his wings. For instance, he could set traps or send minions to attack Percy whenever he tries to fly to new places to explore his powers.
Maybe a rival Viking tribe that has a different view on dragons. They see dragons as a threat rather than friends like Hiccup and Valka do. So they try to disrupt the peace that Hiccup and Valka are trying to build between the Vikings and the dragons. They could be constantly causing trouble, stealing dragon eggs or attacking dragons, which would force Hiccup and Valka to take action against 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.
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.