It might be a new character created specifically for the fanfiction. Maybe an alien race that has a vendetta against both the Saiyans and Earthlings and sees the union (even if it's just a meeting) between King Vegeta and Bulma as a sign to start a conflict.
It could be Frieza. Since he has a long - standing enmity with the Saiyans. His presence would create a great threat that King Vegeta and Trunks would have to overcome together. King Vegeta has his own history with Frieza, and Trunks knows how powerful Frieza can be in the future.
There's a possibility that an evil scientist from another planet could be the main antagonist. This scientist wants to use Bulma's knowledge and Cell's power for his own evil experiments. So he tries to break up their partnership and capture them both for his own purposes. He sends his minions after them and creates various obstacles for Cell and Bulma to overcome.
In 'Vegeta meets Bulma fanfiction', the two main characters are Vegeta and Bulma. Vegeta is known for his combat skills and his quest for power. Bulma is known for her intelligence and her ability to create amazing technological devices. Their relationship is the focus of the fanfiction. Sometimes other characters from the Dragon Ball universe, such as Goku or the other Z - Fighters, may make appearances to add more depth to the story, but Vegeta and Bulma are the stars.
Well, Vegeta himself could be seen as the antagonist in the beginning. Since he is the one doing the kidnapping, he's causing distress to Bulma. However, as the story progresses, the real antagonist could turn out to be someone else. For example, another alien race that is after Vegeta and uses Bulma as bait to lure him into a trap. So, it's not so straightforward in a fanfiction.
In this fanfiction, perhaps King Vegeta would be initially suspicious of Bulma. He's from a warrior race and she's an Earthling scientist. He might be intrigued by her technology though, like her capsule corps inventions.
Frieza could be a great main villain. Since he has a history with Vegeta, his appearance could add a lot of drama. He might try to get revenge on Vegeta or use Bulma as a hostage to force Vegeta to do his bidding.
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.
Their relationship might start off rocky. Vegeta is a proud Saiyan and Future Bulma is a strong - willed woman. But as they face common enemies and challenges in the future world, they begin to respect each other. Vegeta might start to see Bulma not just as a weak human, but as a valuable ally. And Bulma would see the growth in Vegeta and start to develop deeper feelings for him.
At first, it might be a relationship based on mutual need. Vegeta needs Bulma's resources and intelligence to get back on his feet, and Bulma is intrigued by this alien. As time passes, they start to understand each other better. For example, Vegeta may start to open up about his past and his Saiyan pride, and Bulma will share her dreams and ambitions with him.
In this fanfiction, Goku has the potential to be stronger. He has mastered different forms like Super Saiyan and beyond. However, King Vegeta is not to be underestimated. He has his own Saiyan pride and power. If it comes down to a fight, Goku's adaptability and continuous growth in power might give him an edge, but King Vegeta's experience and strategic mind could also make it a very close battle.