The Akatsuki members could be antagonists. Since Naruto is the jinchuriki of the Nine - Tails, they would be interested in him. And with Kakashi protecting Naruto, it would create a lot of conflict. For example, Itachi might be sent to capture Naruto, and Kakashi would have to face off against him to protect his adopted son.
There could also be some within the Hidden Leaf Village who oppose Kakashi's adoption of Naruto. Maybe some old - fashioned ninja who think Naruto is too dangerous because of the Nine - Tails inside him. They could try to undermine Kakashi's efforts to raise Naruto properly and might even plot to separate them.
Orochimaru could still be an antagonist. He might see Naruto as a threat especially if Kakashi is training him intensively. His goal of obtaining power could lead him to target Naruto.
Danzo might be an antagonist. He's always been power - hungry and might see Naruto as a threat, especially if Kakashi is guiding and protecting him. Danzo could try to undermine Kakashi's efforts to raise Naruto and might even send his root agents to spy on them or cause trouble.
Perhaps there are some anti - Uchiha elements within the village that are not happy with Naruto adopting Sarada. They could try to undermine her training or create problems for Naruto and Sarada. They might spread false rumors or try to isolate Sarada from the rest of the village.
One possible antagonist could be Orochimaru. He has always been interested in Naruto for his Nine - Tails power, so it would make sense for him to be a threat that Kakashi has to protect Naruto from.
It could be other villages that see the alliance between the Raikage and Naruto as a threat. For example, some of the more power - hungry villages might fear that the combination of the Raikage's strength and Naruto's abilities would tip the balance of power in the ninja world. So, they would try to undermine their relationship or even attack them directly.
It could be some rogue ninjas who target Sakura because of her relation to Kakashi. They might think they can use her to get to him or extract information from him.
The Otsutsuki clan members other than Kaguya could be antagonists. Since Kaguya is training Naruto, they might see Naruto as a threat to their plans.
Another antagonist could be a group within the ninja village itself. Maybe they fear the changes that the child of prophecy might bring. They could be the more conservative elements who don't want to see new ideas or powers emerging. They might try to undermine the child's training or spread false rumors about them. For example, they could claim that the child is a danger to the village, turning public opinion against the child.
In this fanfiction, Voldemort or his remaining followers could be major antagonists. They wouldn't like the idea of Harry having new allies in Percy and the demigod community. From the Percy Jackson side, the Titans or some of the more malevolent gods might not be happy with the alliance either. They could try to break it up or cause trouble for the two boys, seeing them as a potential force against their own plans.
Sasuke could be a side character. Since Naruto is getting special training from Kakashi, Sasuke might feel jealous or motivated to train harder himself. Sakura could also be there, perhaps she tries to learn from Kakashi's training methods to help Naruto better. Another potential side character could be Iruka, who might check on Naruto's progress from time to time and offer some words of encouragement.
Well, the plot could revolve around Kakashi providing Naruto with a more stable family - like environment. He might cook for Naruto, help him with his studies at the ninja academy in a more hands - on way, and also be there to support Naruto emotionally when he has nightmares about being alone. This could lead to Naruto growing up more confident and perhaps even developing new skills faster as he has someone constantly guiding him.