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.
Well, it could be that Goku somehow ends up in the Naruto universe and has a son with a female character there. And this son has both the powers of a Saiyan like super strength and the ability to use chakra for jutsus. Maybe he's an outcast at first because of his strange powers but then becomes a hero.
The theme of femininity in the context of a powerful warrior like Goku is also quite common. Some fanfictions might explore how a female Goku balances her strength with traditional feminine qualities or how she defies the expectations of what a female Saiyan should be like. Also, there could be themes related to love and romance, especially in relation to characters she has a connection with in the original series, but with a new twist due to her being female.
In some 'oc son of goku fanfiction', a common plot is the OC's struggle with his identity. He is the son of Goku, a great hero, but he wants to make a name for himself. So he might embark on a quest to find a new form or a new fighting style that is all his own. Also, there could be a plot where he has to deal with the jealousy of other Saiyans who feel he gets special treatment due to his father.
One popular 'ash son of goku fanfiction' story could be one where Ash inherits Goku's Saiyan powers and uses them to battle in the Pokémon world. For example, he might use Super Saiyan strength to overpower really tough Pokémon or trainers.
Well, it could be about Naruto somehow obtaining the power of Son Goku. Maybe there's a strange jutsu that fuses their powers. Naruto could use Son Goku's strength in battles, like having his speed and ki blasts along with his own ninja skills.
A rogue ninja who has made a pact with dark forces. He wants to use Goku's son to open a portal to the Saiyan planet and then invade it. He would be constantly trying to manipulate the situation to get Goku's son to do his bidding. He has a team of minions who are also skilled in both ninjutsu and some dark form of ki - like powers.
Well, it could be a really wild story. Harry might inherit Goku's super strength and fighting skills. He could use his magic in combination with Goku - like physical abilities. For example, he could use a spell while flying at super - speed like Goku. It would change the whole Hogwarts dynamic as he would be far more powerful than the other students.
The main characters also include Ash's mother in some fanfictions. She might be worried about Ash's newfound powers and his connection to Goku. In the Dragon Ball universe, Kami could be involved. He could sense Ash's unique existence as a Pokémon - Saiyan hybrid and either try to help him or be wary of his potential. And of course, there are always the various villains from both universes that could be a threat to Ash in these fanfictions.
It could be Orochimaru. He's always after power and seeing Naruto with Son Goku's power would make him a prime target. He might try to extract the power from Naruto for his own evil experiments.