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.
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 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.
One common theme is power growth. Just like in the original Dragon Ball series, in these fanfictions, both Goku and the oc saiyan usually strive to become stronger. They might train together or learn from each other's techniques.
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.
Friendship is also a big theme. The OC and Goku usually form a bond. They rely on each other in battles and in their personal growth. For instance, they might learn from each other's mistakes and support each other when facing difficult opponents.
One popular trope is the 'Powerful OC' trope. In many Goku x OC fanfictions, the OC is often created with unique powers that either complement or rival Goku's. Another is the 'Training Partner' trope, where the OC becomes Goku's training partner and through their training sessions, a relationship develops. And the 'Saved by Goku' trope is also common, where the OC is in some sort of danger and Goku comes to the rescue, which then leads to their relationship evolving.
One popular Goku and female OC fanfiction could be where the female OC is a powerful warrior from a different planet. She meets Goku during a universal tournament and they team up. Their relationship starts as comrades in battle but slowly develops into a deeper friendship as they learn about each other's cultures and fighting styles.
One great 'goku and oc fanfiction' is 'A New Saiyan in the Universe'. It has an interesting plot where the OC is a Saiyan from a lost colony. The OC's journey to meet Goku and their adventures together are really exciting.