It could be a rogue scientist - ninja who has been experimenting with toxins and wants to use them to take over the ninja world. This antagonist would see Naruto's new - found bond with toxins as a threat to his plans.
There could be a powerful dark spirit that thrives on toxins. It has been influencing certain events in the ninja world from the shadows. Once Naruto starts to bond with toxins, it sees him as a target. This dark spirit would be the main antagonist, using its control over toxins and its minions to try and stop Naruto from fully understanding and using his new bond for good.
It could be an old ninja who was once banished from Konoha for using dark jutsu and now wants revenge. He might see Naruto as a symbol of the village he hates and tries to corrupt him by luring him into the darkness.
One possible plotline could be Naruto developing a unique power related to toxins and having to learn to control it. Maybe he forms an unexpected bond with a character who has knowledge of toxins, like a rogue ninja. This bond could lead to him using toxins in a new and unorthodox way in battles.
In some cases, it could be a jealous Hogwarts student. This student might be envious of Harry's special bond with the dragon. They could try to undermine Harry and the dragon's relationship, maybe by spreading false rumors or trying to get the dragon in trouble with the school authorities. This student could be working with some dark forces indirectly, as their envy makes them vulnerable to being influenced by the wrong side.
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.