Family and identity. These fanfics usually explore how the brother fits into the Saiyan family. Is he as noble as Goku? Or does he have a different view on their Saiyan heritage? It's all about self - discovery.
Another common theme is redemption. Maybe the brother starts off on the wrong foot, or has been influenced by bad forces. The story then becomes about his journey to redeem himself and prove his worth to Goku and the others.
Power and training. Since Goku is known for his power - seeking, his brother in fanfics often has a similar drive. They might train in unique ways or discover new forms of power.
The theme of love against all odds is also quite prevalent. Given their different origins and the fact that Goku is already married in the original story, their relationship in fanfics has to overcome many obstacles. This could be social, moral or even physical (like dealing with other powerful characters who oppose their union), but they still find a way to be together, which makes for an interesting read.
One common theme is growth. Both caulifla and goku are Saiyans, so fanfics often focus on their growth in power, like learning new techniques.
Friendship and mentorship are also common. Goku is often seen as a mentor figure to Caulifla in these fanfics. They form a bond through training and fighting side by side. Another theme could be the exploration of different universes. Given that they are from different universes in the Dragon Ball series, fanfics may play around with the idea of them exploring each other's worlds and cultures.
Another theme is power transfer or inheritance. Some fanfics might have Bardock passing on some hidden power or knowledge to Goku. This could be related to their Saiyan abilities or some special technique that only their family line can access. There is also the theme of redemption in some cases. If Bardock is brought back in a fanfic, he may seek to atone for his past actions through his relationship with Goku.
One common theme is the discovery of love. Goku, being somewhat naive in matters of the heart, gradually realizes his feelings for Launch. Another theme is adventure. They often go on quests together, which strengthens their bond.
One common theme is rivalry - turned - friendship. Their long - standing rivalry in the original series is often explored in fanfics as a starting point that then evolves into a deeper connection.
One common theme is the clash of different cultures. Since Goku is from a very different world, when he's in a crossover romance, his Saiyan culture often clashes with the other character's world. For example, his straightforwardness in expressing emotions might be different from the more reserved nature of some other characters.
Love and family are very common themes. Their relationship as husband and wife, and as parents to Gohan and other children is often explored.
One common theme is redemption. Since Broly is often portrayed as a villain initially, many fanfics focus on his journey towards redemption with Goku's help. For example, Goku might show Broly a different way of using his power, leading Broly to change his ways.
Love and family are very common themes. Their relationship is often depicted as a strong bond that withstands Goku's adventures and battles. Chichi is usually shown as the stable presence in Goku's life, and their love story is central.