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.
One common theme is the exploration of Goku's vulnerability. Usually, he's seen as this super - strong character, so when he's hurt, it shows a different side of him. Another theme is the bond between him and his friends and family. They often come together to help him when he's injured.
One common theme is the power - dynamic. It often shows how a female character interacts with Goku's extreme power. For example, she might be in awe of it at first and then strive to reach a similar level.
Another theme could be the challenges Goku faces during the pregnancy. Given that he's a powerful Saiyan, being pregnant might bring out different physical and emotional aspects that are not typically associated with his character. It could also involve how he balances his training and the impending arrival of the baby.
A common theme is training. Since Goku is a master of martial arts, many fanfictions focus on him training Caulifla, like in 'Training Days'. It could be about her learning new forms or just improving her basic skills.
One frequent theme is power - up. Given that Goku is known for his various Super Saiyan forms and Caulifla is also a strong Saiyan, fanfictions like to show them achieving new levels of power, either through training or facing a powerful foe. Additionally, the theme of respect is also common. As they witness each other's strength, they gain respect for one another, which is often depicted in these fanfictions.
Power -ups are common. Since both Goku and Naruto are about getting stronger. So in fanfictions, they often find new ways to increase their power, like Naruto learning to use ki or Goku getting some ninja - like abilities.
One common theme is mentorship. Goku often takes on the role of a mentor to Bulla, guiding her in her martial arts journey. Another theme is friendship. They form an unlikely friendship which is explored in many stories.
The theme of legacy is quite common. Goku is a legendary figure, and in fanfiction, his legacy - what he leaves behind for future generations - is often explored. Also, the theme of redemption can be found. Maybe Goku made a mistake in the fanfiction's plot, and he has to redeem himself. And the concept of the hero's journey is also there. Goku's path as a hero, with its challenges, growth, and revelations, is a recurring theme in serious Goku fanfiction.
Love and family are very common themes. Since they are husband and wife and have a son, many fanfictions explore how they deal with family life together.