If Barry kidnapped Supergirl in fanfiction, it could be a really interesting plot twist. Maybe Barry has been affected by some sort of red kryptonite - like substance that makes him act out of character. This would lead to a conflict between the Flash and Supergirl's friends and allies. They would be confused as to why Barry would do such a thing.
One of the best places to find Flash and Supergirl fanfiction is Archive of Our Own (AO3). It has a large collection of fan - created stories that cover all kinds of scenarios and relationships between the two characters. The site is well - organized, making it easy to search for specific tags or themes related to Flash and Supergirl.
One great Flash and Supergirl fanfiction is 'The Team -Up of Legends'. It shows an alternate universe where they have to work together against a powerful new villain. The story does a great job of portraying their different powers and how they complement each other in battle.
One idea could be that a powerful villain threatens both their cities simultaneously. Supergirl and The Flash have to team up, and in the fanfiction, their different powers and personalities could lead to some interesting interactions. For example, Supergirl's strength and The Flash's speed could be combined in unique ways to defeat the villain.
In this fanfic, the plot could be really interesting. Since it's a crossover, there could be a unique blend of the Supergirl and Flash universes. Kara's pregnancy might be a catalyst for a big change in the story. For example, it could lead to a new form of power or ability within Kara that is related to motherhood. Flash could be the one to first notice the changes in Kara's powers because of his quick thinking and observational skills. The story might also involve a race against time as they try to protect Kara and the unborn baby from a villain who wants to use the baby's potential powers for evil purposes.
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.
The characters would have a range of reactions. Barry (Flash) would probably be very protective. He's a hero who helps people, and Kara is his friend. He'd likely offer his speed and any resources he has to keep her safe. Kara's friends on Supergirl's side, like Alex, would be shocked at first but then become very supportive. They might start planning how to protect Kara and the baby from all the threats they face in their world.
Well, Cisco Ramon from The Flash would be a great addition. He could use his tech skills to create some new gadgets or figure out ways to counter the villains' powers. Also, Martian Manhunter from Supergirl's side. His shape - shifting and telepathic abilities would add a whole new dimension to the story.
One interesting storyline could be a team - up against a powerful new villain that neither The Flash nor Supergirl can defeat alone. They have to combine their powers and strategies in a way they've never done before.