They could have a mentor - mentee relationship. Gohan, being a powerful Saiyan, could teach Bulma about the Saiyan ways, like their fighting techniques and their strong sense of honor. Bulma, in turn, could mentor Gohan in science and technology, which would be useful for Gohan in his battles.
The plot might involve Bulma creating a new device that has some unexpected side effects. Gohan then comes to help her fix it. Along the way, they might encounter some old enemies or new threats. And perhaps through this experience, they develop a deeper understanding and friendship. For example, Bulma could be working on a time - travel device and accidentally sends them both to a different era, where they have to rely on each other to get back home.
Well, in a fanfiction, their relationship could be one of mutual respect. Gohan, being a powerful warrior, might turn to Bulma for her technological expertise. Maybe Bulma could create new training equipment for Gohan. And Gohan, in return, could protect Bulma's research facilities from any threats. It could also be a more of a mentor - mentee relationship in some aspects, with Bulma guiding Gohan on how to use some of her inventions effectively in combat situations.
It's interesting because it allows for new storylines in the Dragon Ball world. Since Gohan and Bulma don't have a typical romantic or overly close relationship in the original series, fanfics can explore uncharted territory. For example, they could go on adventures together, solve mysteries, or support each other in ways that we don't see in the main story. It gives a fresh perspective on the characters and the Dragon Ball universe as a whole.
Maybe in this fanfiction, Bulma creates a new device that has an unexpected side effect on Gohan. It could start to make him see Bulma in a different light. They then go on adventures together, facing various threats. As they fight side by side, their relationship blossoms. Gohan starts to realize that Bulma is not just an old family friend but someone he has strong romantic feelings for.
Often, it starts with mutual respect. Gohan respects Bulma's intelligence and her ability to create amazing things. Bulma respects Gohan's power and his kind heart. As they spend more time together, they start to understand each other better. For example, in a fanfic where they are stuck on an island, they have to rely on each other for survival, and this shared experience brings them closer.
In a fanfic, Future Gohan and Bulma could embark on a journey to find the Dragon Balls. Bulma hopes to use the Dragon Balls to wish for the destruction of the Androids. Along the way, they face many challenges and obstacles. Gohan protects Bulma from various threats, and in the process, they learn more about each other's strengths and weaknesses. This could also be a story of self - discovery for both of them, as Gohan tries to come to terms with his role as the last hope for the world and Bulma tries to accept that she may not be able to save everyone with her inventions.
They could team up to invent something amazing. Bulma has the brains and Gohan has the power and knowledge of the Saiyan abilities. Maybe they create a new type of energy shield that can protect the whole planet from any kind of attack.
They could have adventures where they explore new planets. Gohan's curiosity as a Saiyan and 18's fearless nature would make them great explorers. Maybe they'd discover a new form of energy on an uncharted planet that could help Gohan become even stronger or give 18 some new abilities.
A possible relationship could be a friendly rivalry. They are both powerful in their own ways. Goku is a Saiyan with incredible physical strength and combat abilities, while Android 21 has her own set of unique powers. They might constantly challenge each other to friendly spars, which would help them both improve their skills and also deepen their understanding of each other.
A mentorship relationship could be explored. Bulma, with her knowledge of technology and the world outside of martial arts, could mentor Chi Chi in certain areas. For example, teaching her about new inventions or different cultures. Chi Chi, in turn, could teach Bulma about discipline and family values.