Another possible antagonist could be a new, unknown enemy who is interested in Saiyan genetics. Since Bulma is carrying a Saiyan - human hybrid, this new foe could try to kidnap her to study the fetus. This would force Vegeta to go on a rescue mission while also protecting Bulma from harm.
Frieza or his remnants could be antagonists. They might see the unborn child as a potential threat in the future, especially if it has strong Saiyan powers. So they could plan to attack Bulma to prevent the birth.
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.
There could be a rogue Saiyan who has different ideals from Vegeta. This Saiyan might think that the baby should be raised in a more traditional, warrior - like way and try to take the baby from Bulma and Vegeta. Or, an evil scientist who wants to experiment on the baby's unique Saiyan - human DNA could be the main antagonist, creating all kinds of challenges for Vegeta and Bulma to protect their child.
Their relationship would likely become more intimate and committed. With a baby on the way, Vegeta would probably be more protective of Bulma. He might show his love in more practical ways, like making sure she has everything she needs.
Sometimes new, original antagonists are created for dark fanfiction. They could be powerful demons or shadowy figures from Vegeta's past on Planet Vegeta. These antagonists might have vendettas against Vegeta and see harming Bulma as a way to get revenge. They can bring in elements of mystery and horror to the story.
Perhaps it could be the school bully. This bully might be jealous of Vegeta's strength and popularity among some students, and also of Bulma's intelligence and charm. So, he tries to create problems between them, like spreading false rumors or trying to sabotage their school projects.
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.
Also, Dr. Briefs, Bulma's father, could be in the story. He might be helping Bulma with any scientific aspects related to her pregnancy, or just being a supportive father figure during this time. And perhaps some of the other Z - Fighters could make appearances and have their own reactions to the news of Bulma's pregnancy.
Well, finding a great Bulma and Vegeta pregnant fanfiction can be a bit of a hunt. You could also check out some Dragon Ball fan forums where fans might discuss and recommend their favorite fanfictions in this category. Keep in mind that different writers will have different takes on their relationship during this time, so you might have to read through a few to find one that really suits your taste.
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.
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.