One reason could be to create a huge plot twist. It's a very unexpected and controversial idea that can immediately grab the reader's attention. Another reason might be to explore different emotional states. For example, showing how Vegeta would react to such a betrayal and how Goku would deal with the moral implications of being involved in such a situation.
In some fanfics, there might be such out - of - character scenarios for the sake of creating drama or exploring different relationship dynamics. But in the original Dragon Ball series, this would never happen as the characters have their established relationships and personalities. Bulma is with Vegeta and Goku has his own family and relationships that are very different.
One possible reason could be a radical change in Bulma's character. Maybe she's been corrupted by some dark force or has some deep - seated grudge that has been building up over time. But again, this is very uncharacteristic of her normal self.
In some fanfics, Bulma might be ignoring Vegeta because she is focused on her own scientific projects. She's always been a brilliant scientist, and if there's a major discovery or problem she's dealing with, she could be so absorbed in it that she has little time for Vegeta. It could also be related to the way Vegeta has behaved. If he's been too overbearing or made some mistakes in their relationship, Bulma might choose to ignore him as a form of punishment or to make him realize his errors.
One possible plotline is that they are on the rooftop to escape the chaos of the city below. They find peace and quiet there, and it becomes their special place. They might start reminiscing about their adventures together and how far they've come as a couple.
In the fanfic, it could be that Bulma has fallen out of love with Vegeta. Maybe she has met someone else who seems more compatible with her. This new person could be more in tune with her technological interests or simply be more emotionally available. Or it could be that an event has occurred that has made Bulma see Vegeta in a different, less favorable light. For instance, if Vegeta made a decision during a battle that put their family at risk without consulting her, it could be a breaking point.
One possible plot could be centered around how Vegeta adjusts to the idea of becoming a father. He's a proud Saiyan warrior, so he might initially struggle with the more nurturing aspects. For example, he could be shown trying to train while also being worried about Bulma's well - being during her pregnancy. Maybe he starts to research Saiyan pregnancies to make sure everything is going as it should.
One interesting plotline is when Bulma's inventions are used to enhance Goku and Vegeta's powers in unique ways. For example, she might create a device that helps them control their ki better. Another is when Goku and Vegeta have to work together to protect Bulma from a new and powerful enemy. It shows their growth as characters who can put aside their differences for someone important to them.
Another storyline could be that Bulma creates a new device that has some unexpected side effects. Goku is the only one who can help her fix it, and during this process, they accidentally get sent to a parallel universe. There, they have to work together to find a way back home, all while their relationship blossoms in this strange new world.
One challenge could be protecting the baby from their enemies. Vegeta has made many foes over the years, and they might target the baby. Also, Vegeta's Saiyan nature might make him push the baby too hard in training, while Bulma wants a more relaxed approach. This difference in parenting styles could cause some friction between them.
One reason could be the constant threat to the world in the Dragon Ball universe. She might feel helpless as a non - fighter, seeing her friends and family in danger all the time. For example, when powerful villains like Frieza or Cell were on the loose, she could be depressed about her inability to contribute more directly to the fight.