One interesting plot could be how Vegeta adjusts to the idea of becoming a father. He's a proud Saiyan warrior, and this new role might be a big challenge for him at first.
Another plot could involve the reactions of their friends. Goku might be really excited and a bit clueless about the whole pregnancy thing. And Chi - Chi could offer Bulma some advice based on her own experiences of raising Gohan and Goten.
Maybe in the fanfiction, Bulma has to deal with her own insecurities during the pregnancy while also handling Vegeta's overprotective nature. For example, Vegeta might not want her to continue working on her inventions for fear of something happening to her or the baby.
A possible plot could also revolve around a threat to the Earth where Vegeta and Bulma's relationship is put to the test. Vegeta has to go and fight, and Bulma has to deal with the fear of losing him. This could show the depth of their bond and how they support each other through tough times.
A great plot could be how their relationship affects the rest of the Dragon Ball gang. Goku might be a bit confused at first, and Chi - Chi could be either supportive or a bit jealous. And then there's the whole dynamic of Vegeta becoming more involved in Earth - based family life, which is a huge change for him.
One interesting plot could be Vegeta trying to understand human emotions better because of his relationship with Bulma. He might struggle with expressing his love in a more 'human' way, which could lead to some really sweet and comical moments.
One common theme is Vegeta's struggle to adapt to the idea of becoming a father. He's a Saiyan warrior, so the concept of family in a more domestic sense is new to him.
Another common plot is the 'time - travel' scenario. Given the time - travel elements in DBZ, some fanfictions might send Vegeta or Bulma back in time. This can lead to interesting situations where they have to interact with their younger selves or change certain events in the past. It also gives a chance to explore their relationship in a different timeline.
A third fanfiction concept could be about how they prepare for the baby's arrival. Vegeta, not knowing much about Earthling baby things, has to learn. Bulma tries to teach him, and there are a lot of cultural clashes and misunderstandings at first. But as time goes on, they work together to create a perfect nursery and get ready for their new family member.
The fanfiction could focus on the challenges they face during the pregnancy. For example, Bulma might have some health scares that make Vegeta worry a great deal. He could struggle with his emotions, not knowing how to handle his fear of losing her or the baby. Also, there could be a sub - plot about how the news of Bulma's pregnancy affects the political situation in the Dragon Ball universe. Some enemies might see it as an opportunity to attack while some allies might offer more support than ever.
One interesting story could be about their adventures together in a parallel universe. In this fanfic, Vegeta and Bulma might face new enemies that they have to defeat while also dealing with their relationship issues. Maybe they discover a new power that only their combined efforts can control.
In Vegeta and Bulma fanfiction, a great part is seeing how their different personalities mesh. Vegeta is all about power and pride, while Bulma is intelligent and resourceful. Fanfictions might show how they balance each other out. For instance, Bulma could use her smarts to come up with strategies in battles that Vegeta is involved in. Also, their cultural differences, Vegeta being from a Saiyan culture and Bulma from Earth, can lead to interesting storylines. Writers can explore how they deal with these differences and grow closer because of them.
One interesting plot could be a time - travel related one. Vegeta and Bulma accidentally get sent to the past. There, they have to deal with the different versions of their friends and enemies. They also have to find a way to get back to their own time while not disrupting the timeline too much. This would be full of adventure and also explore their relationship in a different historical context.