It's possible that the victim is a new character introduced in the fanfiction. This character could have some unique ability or knowledge that the Flash believes is too dangerous to be left in the wrong hands, so he kidnaps them to keep it safe, which then sets off a chain of events in the story.
It could be Iris. Since she's an important character in The Flash's story. The Flash might be forced to kidnap her for some strange reason, like to protect her from a threat that he can't explain to her directly.
It could be a young and innocent bystander. Maybe someone who was just in the wrong place at the wrong time. For instance, a small child who happened to be near where Will and Phobos were planning their kidnapping and was grabbed instead of their intended target.
A young journalist who has been writing articles about Kaito Kid and his heists. Kaito Kid kidnaps the journalist to control the narrative. He wants to make sure that the journalist doesn't publish any more information that could give away his next moves or expose his secrets.
The victim could be Alec Lightwood. Since Magnus and Alec have a complex relationship, a fanfiction could explore what would happen if Magnus kidnapped Alec, perhaps out of some miscommunication or a threat to Alec's safety that Magnus thought he could only handle by taking Alec away. Another potential victim could be a young, inexperienced warlock. Magnus might kidnap the warlock to teach them about their powers in a more secluded environment away from the prying eyes of the Shadowhunters.
A young boy who is the chosen one to bring about a new era of warmth could be the victim. Jack Frost kidnaps him to prevent the change in the world's climate that would come with the boy's power.
Well, it could be that the Flash, perhaps under some mind - control or in a desperate situation, kidnaps someone important in the fanfiction world. Maybe he's trying to prevent a greater evil and thinks this is the only way.
It could be a new, powerful villain who wants to make a name for themselves by taking out two well - known sidekicks. This villain might have some unique powers or abilities that make it difficult for Robin and Kid Flash to escape.
I'm not entirely sure specifically what 'flash kidnapped fanfiction' refers to without more context. It could be a fan - created story in which a character named Flash is kidnapped within the realm of fanfiction. Fanfiction often takes existing characters and creates new, sometimes wild, storylines.
In the fanfiction, they might be kidnapped by a supervillain who wants to use them as bait to lure Spider - Man. Maybe the kidnapper thinks Flash has some connection to Peter Parker (aka Spider - Man) and plans to expose his identity through Flash.
It could be Clary. Since she is a very important figure among the Shadowhunters and has a lot of power, Magnus might kidnap her if he thought it would help him achieve a certain goal. Maybe he needs her unique rune - drawing abilities for a spell.
Another possibility is that Barry was kidnapped himself first and then forced to kidnap Supergirl. The real kidnappers could be a group of meta - humans who want to create chaos by pitting two of the most powerful heroes against each other. Barry would be in a difficult situation, having to do something against his will, and Supergirl would initially be confused and angry at Barry until they both figure out what's really going on.