It could be a long - lost Uchiha clan member who has a killer instinct amplified by a dark jutsu. This character has a vendetta against the Hidden Leaf for past wrongs done to the Uchiha. They use their Sharingan in very cruel ways, not hesitating to kill even innocent bystanders to achieve their goals.
It could be a rogue ninja who has also discovered a dark version of the ultra instinct. This ninja uses his power for evil, like taking over villages and enslaving people. He would be a major threat to Naruto and his friends because his power is so similar yet so different from Naruto's ultra instinct - like powers.
Maybe it could be a new dark wizard or witch who has a unique power that confounds Harry's rational thinking. This antagonist could be someone who uses mind - games and illusions, making it difficult for Harry to use his normal knowledge to fight back. So, he has to rely on his instincts more than ever to figure out their true intentions and defeat them.
Voldemort could still be the main antagonist. Even with Harry having the ultra instinct ability, Voldemort's dark magic and his army of Death Eaters would pose a significant threat. His cunning and thirst for power would make him a formidable foe against Harry's new power.
Another possibility is a rogue Saiyan from an alternate timeline. This Saiyan could be jealous of Goku's status as the strongest Saiyan in his own timeline. So, they use time travel to come to Goku's timeline and try to defeat him and take over as the dominant Saiyan.
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.