Maybe a group of rogue ninja who believe that the combination of Naruto and the Kyuubi is a threat to the balance of the ninja world. They aim to separate them by any means necessary, even if it means causing great destruction. They might have some dark jutsu that can target the bond between Naruto and the Kyuubi.
It could be an ancient shinobi who once tried to control the Kyuubi but was sealed away. Now, with Naruto as the new host, he sees an opportunity to take over the Kyuubi's power again.
Madara Uchiha could also be a main antagonist in this fanfiction. He has grand plans for the ninja world and the released Kyuubi could be a key part of his scheme. Since Naruto is dead and can't oppose him, he might try to use the Kyuubi to reshape the world according to his vision. He has the power and the ambition to be a major threat in this situation.
Obviously, the fem Kyuubi herself could be the main antagonist. Since she is the one manipulating Naruto, her actions are causing problems and driving the conflict in the story.
It could be an evil ninja who wants to capture the fem Kyuubi for her power. He might see the unique bond between Naruto and the fem Kyuubi as a threat to his plans of world domination.
It could be Orochimaru. He's always been interested in power and the secrets of the Kyuubi. With Naruto releasing the Kyuubi fuuinjutsu, Orochimaru might see this as an opportunity to either steal Naruto's power or use him for his own evil experiments.
It could be other powerful ninja clans who see the fem Kyuubi's possessive behavior towards Naruto as a threat to their own power and influence. They might try to separate them.
It could be another powerful entity that is secretly manipulating the situation. Maybe there is a dark shinobi or a hidden organization that wants to see the Kyuubi and Naruto at odds. They could be the ones who set in motion the events that lead to Naruto scaring the Kyuubi. For example, they might have given Naruto a false power or false information that is causing the Kyuubi to be scared, all in an attempt to further their own evil plans. This entity would then be the main antagonist as they are the real source of the conflict.
It could be Orochimaru. He might want to use Ino's connection with the Kyuubi to his own advantage. Maybe he plans to extract the Kyuubi's power through Ino's mental link with it.
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.