In 'oc son of goku fanfiction', antagonists can also be from other universes. There could be powerful warriors from parallel universes who come to challenge the OC because they see him as a key to controlling the multiverse or because they want to prove their universe's warriors are stronger. Also, some of Goku's old enemies might have descendants who hold grudges and target the OC as a way to settle old scores.
A group of rogue Saiyans from a parallel universe might also be antagonists. They see Goku's Ultra Instinct as a threat to their own plans of universal domination. They have their own unique powers and battle tactics, making them a formidable foe for Goku and his friends.
One type of antagonist is a mad scientist - type character who wants to experiment on Goku and Fasha to understand their Saiyan powers. This character might create all sorts of dangerous monsters to try and capture them. Another could be a powerful demon from another dimension who is drawn to their power and wants to consume it to become even more powerful.
The witches in the story can also be antagonists. They may have their own motives and prophecies related to the baby. They could try to manipulate the situation to their advantage, perhaps by using dark magic to influence the OC or the baby's fate.
Cell could also be an antagonist. He has an interest in powerful Saiyan genetics. The baby, being related to Goku, would be of great interest to him. He might try to absorb the baby to gain even more power, creating a big conflict for the heroes to deal with.
There could be new antagonists created specifically for this fanfiction. For example, an evil scientist who is jealous of Bulma's intelligence and wants to steal her inventions. This new villain could team up with other existing villains in the Dragon Ball universe to create chaos and try to break up Goku and Bulma's marriage.
Often, it could be new villains created specifically for the fanfiction. Maybe a powerful android created by the same scientist who made Android 21 but with a more evil intent.
One common theme is power - up. The OC usually has some sort of unique power or ability that they either learn to control better or gain more of with Goku's help. For example, in many fanfictions, the OC might start with a weak power but end up becoming really strong like Goku.
It could be some of the more powerful villains from both universes combined. Maybe Frieza and Orochimaru team up. Frieza's lust for power and Orochimaru's desire for immortality and knowledge would make them a formidable pair. They would target Naruto because of his potential and Goku because of his strength, aiming to either destroy them or use them for their own evil purposes.
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.
Often, it could be some sort of super - powered being from a different dimension. Maybe a powerful wizard who wants to take over both their universes.