It could be Frieza. Frieza has a long - standing grudge against the Saiyans, especially Vegeta. He might kidnap Vegeta to torture him or to use him as a bargaining chip against the other Saiyans. Another possibility is Cell. Cell might see value in Vegeta's genetic material and kidnap him to extract it for his own evolution.
In a 'Vegeta kidnapped' fanfiction, perhaps Vegeta could be kidnapped by an old enemy seeking revenge. This enemy might be from his Saiyan past. They could be using him to draw out Goku, thinking Goku will come to save Vegeta. Vegeta, being proud, would likely struggle against his captors and try to break free on his own at first.
There's also a chance that it could be a rogue member of a hero organization. They might have gone bad and decided to kidnap Robin for some self - serving reasons, like trying to gain power or access to certain secrets within Young Justice.
Perhaps a jealous and power - hungry magician from Zatanna's own magic - using community. This person might feel overshadowed by Zatanna's growing power and popularity. So, they decide to kidnap her to steal her magic or to simply get her out of the way so they can be the top magic - user in their world.
One possible plot could be that Zatanna is kidnapped by a group of magic - hating villains who want to use her as a bargaining chip against the Justice League. They might hold her in a magically - sealed location, and the Young Justice team has to figure out how to break the seals and rescue her.
Well, one possibility is that the other members of Young Justice immediately start a search and rescue mission. They might use their unique skills and powers to track Robin down. For example, Superboy could use his super strength to break through any barriers in their way, while Miss Martian could use her telepathy to try and sense Robin's location. Another scenario could be that Robin manages to escape on his own using his wits and combat skills. He's been trained well by Batman, so he might be able to outsmart his captors.
If Vegeta was a girl, it might change the dynamics within the Dragon Ball universe. Her interactions with other characters could be very different, and her power might be perceived differently too.
First, develop interesting characters. Make the twins have distinct personalities. For example, one could be brave and impulsive while the other is more cautious and analytical. Then, create a tense and mysterious setting for the kidnapping. Maybe it's an abandoned warehouse on a stormy night. Also, build up the motivation of the kidnappers. Are they after ransom, revenge, or something else? This will keep readers engaged.
The Naraku would be a likely kidnapper. He is always plotting against Inuyasha and trying to get his hands on the Tessaiga. Kidnapping Inuyasha would be a way for him to either steal the sword or find a way to break Inuyasha's spirit so that he can no longer oppose him.
One possible analysis could be that it might explore the complex relationship between Vegeta and Trunks. Maybe in this fanfiction, Vegeta's strict training methods could be seen as 'hurting' Trunks in a sense, like pushing him too hard physically or emotionally. It could also delve into how Trunks copes with that and if it ultimately makes him stronger.