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.
Majin Buu can also be a major antagonist in this type of fanfiction. In a darker take, he could be even more unpredictable and malevolent. His ability to regenerate and his insatiable appetite for destruction could be enhanced, and he might target Bulma specifically to enrage Vegeta and draw him into a fight.
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.
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.
Another character could be Dr. Briefs, Bulma's father. In the alternate universe, he might have a more prominent role in bringing Vegeta and Bulma together. For example, he could be the one who arranges for Vegeta to work on a project with Bulma, which starts their relationship on a more professional note that gradually turns into something more personal.
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.
There could be a group of characters led by a wise elder. The elder has some knowledge about the dark below but not enough, so they all embark on a journey together. Each character in the group might have different skills like combat, magic, or navigation that are useful in the exploration.
Often, Naruto himself can be a main character. He might be exploring the new power of dark fire, either trying to master it for the good of the village or facing the internal struggle of whether to use such a dangerous power. Sasuke could also be a main character. Given his own complex relationship with power and his past, he could be drawn to the allure of dark fire and how it could help him achieve his goals.
Obviously, the Darkwraiths themselves are main characters. They are often depicted with their dark armor and menacing presence. Their actions and motives drive many of the stories.
In a lot of these fanfictions, Acnologia is a common antagonist. He's such a powerful dragon in the Fairy Tail universe that he makes a great villain. Another one could be Zeref. His dark magic and his complex relationship with the main characters can be explored in Nalu fanfictions related to Dragon Cry.
Bulma, on the other hand, is the brilliant scientist from Earth. She's smart, sassy and not afraid to speak her mind. Their different personalities make for an interesting dynamic in the fanfiction.
Well, while Vegeta and Bulma are the main focus, other characters can also play important roles in the fanfiction. For instance, Trunks, their son, might be affected by his parents' relationship development shown through this kiss. Also, friends like Yamcha, who was once Bulma's boyfriend, could be part of the story, perhaps having some reaction to the new development between Vegeta and Bulma.