Perhaps there are some within the village who secretly envy Naruto's power and status. They could try to use Kunou as a means to undermine Naruto. They might spread false rumors about her or try to isolate her from the other villagers, causing trouble for Naruto as he tries to protect his daughter.
There could be external forces, like other villages or rogue ninja groups. They might target Kunou to get to Naruto or because they fear the power she could potentially possess in the future. Maybe they see her as a symbol of a new threat emerging from Naruto's lineage.
One possible antagonist could be some of the traditionalist shinobi in the village who oppose Naruto's new family situation. They might see Kunou as a threat due to her heritage.
Perhaps Naruto discovers some hidden powers or abilities that Kunou has inherited from him, which leads to a whole new adventure. They might go on a journey to understand and control these powers together. Also, Naruto could face challenges from the village elders who are not so accepting of Kunou at first.
The main antagonists could be some of the traditional ninja clans who see Naruto's connection to the monsters as a threat to their power structure. They might fear that the monsters will disrupt the balance of power in the ninja world.
One possibility is that a writer might create a backstory where Naruto's unique chakra or abilities are needed to save Kunou's mother from a great danger. In the process, a bond forms between them and through some unorthodox means, Naruto becomes the father. Another aspect could be that in the fanfiction world, there are new rules and the author wants to explore the combination of Naruto's ninja world elements with the devil - based world of 'Highschool DxD' to make Naruto Kunou's father.
If Naruto is Kunou's father, it would mean a whole new set of powers for Kunou. Combining Naruto's chakra - based powers with the devil - like abilities in 'Highschool DxD', Kunou could be extremely powerful. For example, she might be able to use Naruto's Rasengan technique along with her own devil powers.
Another aspect could be a more personal connection. Naruto, having grown up without his parents for a long time, could understand Kunou's situation in some ways. He might offer her advice on how to deal with the pressures of being a princess and a ninja - in - training. Their meeting could be a turning point in Kunou's life, inspiring her to be more confident and to strive for greater things, like Naruto has always done.
In Percy Jackson father fanfiction, the sea god Triton could be an antagonist. He might be jealous of Percy getting so much attention from Poseidon. The primordial sea gods or goddesses could also be antagonists. They might see Poseidon's relationship with a half - blood as a perversion of the natural order. And then there are always the minor gods who might feel that Percy and Poseidon are getting too much power and try to undermine them.
One potential antagonist could be a group of rogue ninjas who see Sarada's pregnancy as a sign of weakness in the village. They might try to target her to cause chaos. Another could be a jealous rival who doesn't like the attention Sarada is getting due to her pregnancy and her connection to Naruto.
It could be Orochimaru. He might be interested in the baby's unique genetic makeup, thinking he can use it to create more powerful experiments. He has always been obsessed with power and the baby's combination of Naruto and Temari's abilities would be very appealing to him.
The village elders could be the antagonists. They might have a hidden agenda and want Naruto out of the picture, so they imprison him unjustly.
Madara Uchiha could also be a possible antagonist in this scenario. Even if he is dead in the original story, in a fanfiction, his lingering influence or a resurrected form could manipulate Naruto. Madara was always against Konoha's ideals to some extent and might use Naruto as a tool to destroy what he hated.