Lex Luthor could be another antagonist if there is some sort of crossover of universes. Lex might be jealous of Kal - El's power and the fact that he has found a new home with Bulma. He could try to manipulate events in the Dragon Ball universe to turn the Z - Fighters against Kal - El or try to steal his powers for his own evil purposes.
Maybe Cell could also be an antagonist. Cell is always looking to absorb powerful beings to increase his own strength. Kal - El's unique powers would be very appealing to Cell. He might try to target Kal - El when he is still young and not fully in control of his powers, thinking it would be an easy way to gain more power.
One possible antagonist could be Frieza. Since he's always been interested in powerful beings, the presence of Kal - El with his superhuman abilities might draw Frieza's attention. Frieza could see Kal - El as a threat to his plans of universal domination and try to eliminate him.
Well, it could be about Bulma finding Kal - El (Superman as a baby in the DC lore). Maybe she discovers him in some sort of space pod crash on Earth in the Dragon Ball universe. She decides to raise him, and Kal - El starts to grow up with all the advanced technology of Capsule Corp around him.
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.
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.
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.
The main antagonists could be the traditional enemies of Camelot like Morgana. She might see Merlin's act of adopting Mordred as a threat to her plans. Also, some of the knights or nobles in Camelot who are superstitious and believe Mordred is a danger just because of the prophecies could be antagonists. They might try to separate Merlin and Mordred or even harm Mordred.
There could be a rogue Saiyan who has different ideals from Vegeta. This Saiyan might think that the baby should be raised in a more traditional, warrior - like way and try to take the baby from Bulma and Vegeta. Or, an evil scientist who wants to experiment on the baby's unique Saiyan - human DNA could be the main antagonist, creating all kinds of challenges for Vegeta and Bulma to protect their child.
It's possible that some of the more traditional and strict gods could be antagonists. They might view Aphrodite's act of adopting a mortal boy as going against the natural order. These gods could try to separate the boy from Aphrodite, either by tricking the boy or by putting pressure on Aphrodite. There could also be a dark force or a cursed entity in the story that targets the boy because of his connection to Aphrodite, seeing him as a way to get back at the goddess of love.
Hermione Granger and Ron Weasley are also likely to be important characters. Hermione's intelligence and Ron's loyalty can play important roles. They might be initially suspicious of Kal El but then come to trust and rely on him. For example, Hermione could help Kal El understand the complex magic theories, and Ron could be his partner in some magical adventures or battles. Kal El's presence would also add a new dynamic to their friendship with Harry.
The Death Eaters could still be the main antagonists. They might see Fenrir's new alliance with Harry as a threat. Since they once used Fenrir for their dark deeds, they don't want him to turn good and expose their secrets.