Another possibility is a new character created specifically for the fanfic. This character could come from a different planet or dimension and be intrigued by Bulma's intelligence and beauty. They would then set out on a journey to 'conquer' her, which would lead to various interactions with the DBZ universe characters.
Vegeta would be a likely candidate. Given his complex relationship with Bulma in the original series, a fanfic about 'conquering' Bulma could center around him as he tries to better understand and connect with her on a deeper level.
It could also be Yamcha. He has a history with Bulma, and in this fanfic, he might be trying to win her back. He could be going through a transformation of sorts, becoming stronger and more confident to prove to Bulma that he's changed for the better.
Well, perhaps it could involve a character from DBZ having to overcome various challenges to 'conquer' Bulma in a non - violent or more of a friendly competition sense. For example, Goku might have to win a series of tests of strength and intelligence set by Bulma herself.
Vegeta could also be the main character. He has a strong pride and a burning desire to be the strongest. In a 'break through the limit' fanfic, his journey to surpass his own limits and perhaps even overtake Goku could make for an interesting story.
There's a possibility that it's a dark wizard - like being who has created this burning black energy as a weapon. He might have some vendetta against Bulma because of her scientific achievements that he sees as a threat to his magic - based power. And Vegeta gets in his way when he tries to harm Bulma, making them the main targets of this antagonist.
Frieza could be a great antagonist. He has a history with Broly and would be interested in Bulma's technology. He might see Broly's new alliance with Bulma as a threat and try to break it up. Frieza could also want to use Bulma to create powerful weapons or devices for his own evil purposes.
Yamcha could potentially betray Bulma. Even though they have a history together, in a fanfic scenario, he could be jealous of Bulma's growing closeness with Vegeta or her scientific achievements. So, he might do something to undermine her, like spreading false rumors about her work.
In Saiyan fanfics, Vegeta and Bulma's love is often depicted as a complex and evolving relationship. Vegeta, the proud Saiyan prince, initially has a rough exterior. But Bulma's tenacity and unique charm gradually break through his barriers. Their love story can add a lot of emotional depth to the Saiyan - centered fanfic universe.
Bulma would be a main character. Another could be Goku. Even though Goku is more known for his pure heart and fighting skills, a fanfic could explore a scenario where his scent has an unexpected effect on Bulma. It could be a different take on their relationship within the Dragon Ball universe and add a whole new layer of intrigue to the story.
One possible antagonist could be Frieza. Since he's always been interested in powerful beings, the presence of Kal - El with his superhuman abilities might draw Frieza's attention. Frieza could see Kal - El as a threat to his plans of universal domination and try to eliminate him.
In DBZ Vegeta and Bulma fanfiction, there can be alternate universes explored. Maybe in one universe, they meet under different circumstances. For instance, instead of Vegeta coming to Earth as an enemy, he could be an ally from the start. This would completely change the dynamics of their relationship. It could lead to a more romantic and less combative start to their relationship, and the fanfiction could explore how this different beginning affects their future together.
Often, the main character is a self - insert type of gamer who gets transported into the DBZ world. They usually have the ability to use gamer - like mechanics to interact with the DBZ universe.