A group of outcast animal clans could be the main antagonists. They were exiled from the main animal kingdom for their violent and power - hungry ways. Now they have united and plan to take over the entire animal world and overthrow the current Animal King. Naruto gets in their way as he is trying to protect the balance between the human and animal worlds, and they see him as a major threat.
It could be an escaped soul from the King of Hell's domain. This soul might have a grudge against Naruto or the ninja world in general. Maybe it was wrongly imprisoned and now wants revenge by causing chaos in the living world.
It could be an evil ninja from the Naruto world who has somehow gained the power to traverse into the King of Heroes realm. This ninja, with his newfound powers and his thirst for domination, starts causing chaos in both worlds. He wants to use the power of the heroic spirits for his own evil purposes and sees Naruto and the King of Heroes as his main obstacles.
Perhaps an organization similar to Akatsuki. They might see the king of beasts as a powerful tool to achieve their goals, whether it's collecting all the tailed beasts or just ruling the world with an iron fist. They would send their strongest members to try and capture the king of beasts, and Naruto would have to stop them. In this fanfiction, this antagonist organization could have new and more powerful abilities compared to the original Akatsuki in the Naruto series.
Madara Uchiha could also be a great antagonist. Imagine if he came back and thought that the power of the Soul King could be used to achieve his ultimate goal of creating a world under his control. He'd be a formidable foe for Naruto, especially since he has so much experience and power already.
A powerful rogue Shinigami could be the main antagonist. This Shinigami might be jealous of the Shinigami King's power and wants to overthrow him. He sees Naruto as a threat because of his connection to the king and tries to stop Naruto from fulfilling any tasks or missions related to the king. He could also be causing chaos in the living world by releasing dangerous spirits or curses.
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.
Perhaps a group of poachers in the wizarding world who hunt magical animals could be the antagonists. If Harry is living as a rare magical animal, they would be after him. They might not know his true identity but would be attracted to the magical properties he has in his animal form. Harry would then have to use his wits and any remaining magical abilities to avoid being caught by them.
Maybe a powerful demon who has the ability to possess animals. He uses this power to send hordes of possessed animals against Gohan. His goal is to drain Gohan's energy and use it to break free from a magical seal that has been confining him for centuries. He sees Gohan as a threat to his plan because of Gohan's strength and connection to the Dragon Balls.
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.