One way is through joint adventures. If they go on a journey together, they'll be forced to rely on each other more. Goku can protect Bulma from physical threats, and Bulma can use her smarts to solve problems like getting through traps or deciphering ancient texts. This mutual dependence will bring them closer. Also, Bulma could introduce Goku to more of her scientific projects, and Goku could show her new techniques he's learned in his training. As they share these different aspects of their lives, their relationship will grow.
They could start by sharing more personal stories. For example, Goku could tell Bulma more about his time on Planet Yardrat. This would deepen their understanding of each other.
One way is to focus on their shared adventures. For example, when they are on a quest for a new power source or to stop a new threat, they can rely on each other more. Goku can protect Bulma physically, while Bulma can use her intelligence to help Goku strategize. Another way is to explore their past more deeply. Maybe there are some untold stories from when they first met that can strengthen their bond.
Goku's understanding of different universes and races could be deepened in fanfiction. Since Dragon Ball Super introduced the multiverse concept, fanfiction could explore how Goku interacts with and learns from beings from other universes, making him more worldly and less naive in some ways.
To develop Goku Jr's character further, he could be given more complex emotions. For example, showing his fear in the face of truly overwhelming enemies, rather than always being fearless like Goku. This would make him more relatable. Also, exploring his relationships in more depth, like his friendship with Pan. Maybe they have a falling out over different views on how to protect their world, which forces Goku Jr. to grow and understand different perspectives.
At first, they were just partners in the search for Dragon Balls. Bulma was attracted to Goku's unique naivete and strength in a way.
One aspect that fanfictions might play with is the contrast between their personalities. Goku is simple - minded and focused on fighting and getting stronger, while Bulma is intelligent and more worldly. This contrast can lead to interesting character dynamics in the fan - created stories. Maybe in some fanfictions, Bulma helps Goku understand more complex aspects of the world outside of fighting.
To develop Cooler further, he could be given a redemption arc. Maybe he starts off as a villain but gradually realizes the error of his ways. He could befriend some of the younger Z - fighters who look up to him despite his past. This would force him to be a better role model and in the process, he would change and become a more complex and interesting character.
In the fanfiction, their relationship might start off as a bit of a shock to both of them. But gradually, they begin to respect each other more. Goku starts to see Bulma's intelligence and how it can benefit their family and the world. Bulma, on the other hand, comes to admire Goku's unwavering sense of justice and his strength. They communicate more often, sharing their thoughts and dreams. For example, Goku might tell Bulma about his Saiyan heritage in more detail, and Bulma might share her latest scientific ideas with Goku. This sharing helps them grow closer together.
They could go through a period of misunderstanding. Maybe Gohan makes a decision about his future that Chichi doesn't initially understand. Through communication and some trials, they grow closer as they learn more about each other's perspectives.
They could also develop their relationship through training. If Vegeta takes a more hands - on approach to training Trunks, not just as a student but as his son, they could bond over the shared experience of pushing their limits. They could learn new techniques together and grow closer as a result.
One possible scenario could be Goku and Bulma going on a quest together to find a new type of Dragon Ball that has unique powers. They might face various challenges along the way, like dealing with new villains who also want the Dragon Ball. Bulma could use her intelligence to outwit the enemies while Goku uses his strength to fight them off when necessary.