A powerful demon or dark entity. This entity could be using the princess's death as part of a larger, evil plan. For example, her death could have been a sacrifice to open a portal to a dark dimension or something, and her resurrection would disrupt that plan. So, it would do everything in its power to stop Naruto and the others from resurrecting her.
It could be a dark wizard or sorcerer who was the one responsible for the princess's death in the first place and doesn't want her to be resurrected. They might have their own dark motives like wanting to rule the princess's kingdom or gain some powerful magic from her death.
There could be a group of corrupted Quincy who view Ichigo's resurrection as an abomination. They believe that his power goes against the natural order of things. So, they band together to try and eliminate him. They might have some new and dangerous weapons or techniques specifically designed to counter Ichigo's resurrected powers. This group could be led by a very powerful Quincy who has a personal vendetta against Ichigo or the Soul Reapers in general.
It could be a power - hungry warlord who wants to conquer both Naruto's land and Princess Kuyoki's kingdom. He sees their growing alliance as a threat and tries to stop them at every turn.
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.
Bowser could be the main antagonist as usual. He might be jealous of Princess Peach's newfound popularity during the Showtime and decides to crash the event and kidnap her once again, leading to a rescue mission by Mario and his friends.
The main antagonist could be an evil knight who has a vendetta against dragons. When the princess turns into a dragon, he sees her as a prime target and tries to slay her, believing all dragons are evil. He doesn't care that the princess was once human and is only focused on his own prejudiced views.
It could be an envious noble. This noble might be jealous of Sakura's position as a princess and her popularity among the people. So, they try to spread false rumors about her and plot to dethrone her.
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.