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.
One common theme could be the power of friendship. Naruto is all about making bonds, and in a fanfiction with a princess resurrection, he might form a strong bond with the princess as they face challenges together. Another theme could be sacrifice. To resurrect the princess, characters might have to give up something valuable. And also, the theme of redemption might be present. Maybe the princess had some past mistakes or her kingdom did, and through the resurrection and the story's progression, there is a chance for redemption.
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 an ancient evil spirit that was sealed away long ago but is awakened by the emergence of the Adamantium bloodline. This spirit believes that the power of the bloodline can be used to break free from its final seals and wreak havoc on the world.
It could be a dark ninja who wants to control the power of the fire dragon for his own evil purposes. He might try to steal the dragon's chakra or use it to destroy villages.
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.
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.
A group of rogue ninja who are hunting nekoshou for their special powers could be the main antagonists. They see Naruto's newfound connection with the nekoshou as a threat and try to eliminate him and the nekoshou he is protecting.