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.
Well, one possible plot could be Naruto seeing Sarada in a difficult situation, perhaps without proper guidance from Sasuke who is often away. Naruto, having a big heart and remembering his own lonely childhood, decides to take her under his wing. He could teach her some of his signature jutsu and also instill in her the values of hard work and friendship that he learned from Iruka and his friends.
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.
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.
Well, if Naruto adopts Sarada in a fanfiction, it could start with Sarada facing some difficulties in her relationship with her mother, Sakura. Naruto, having grown up without parents himself, empathizes with her. He adopts her and takes her on adventures outside the village. This would expose Sarada to different cultures and ninja techniques. Along the way, they would form a deep bond, and Sarada would become more like Naruto in her positive attitude towards life and determination to protect the village.
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.
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.
In the Naruto world, it could be Orochimaru. He might be interested in Naruto's newfound Pokémon - related powers and try to capture him to experiment on him. Orochimaru is always after power, and Naruto's unique abilities could be very appealing to him.
Another possibility is that some rogue ninja who have a vendetta against Hanzo might target Naruto as a way to get back at Hanzo. They could see Naruto as Hanzo's weakness and try to harm or kidnap him, which would then lead to a series of battles and confrontations in the fanfiction.
One possible main antagonist could be Frieza. Imagine if Frieza somehow found his way into the Naruto universe. He would be attracted to the chaos and power struggles there. Goku and Naruto would have to team up to stop him from taking over the Naruto world. Frieza's cold - blooded nature and his desire for power would put him at odds with our heroes. His powerful abilities would be a huge threat to both Naruto and Goku, especially since Naruto is still learning about the new powers Goku is teaching him.
The child could be an orphan from a small, destroyed village that Naruto comes across during one of his missions. This child might have lost their family to a powerful enemy and Naruto feels a sense of responsibility to take them in.