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 inspiring Supergirl story is her fight against powerful villains to protect Metropolis. She uses her super strength, flight, and heat vision to keep the city safe. For example, when a group of alien invaders threatened the city, Supergirl was the first to stand up and fight them off, showing great courage.
When Supergirl is hurt in fanfiction, it can show her as more human - like. Instead of being this all - powerful being, her pain makes her relatable. For example, if she's injured and has to rely on others for help, it shows her vulnerability and the importance of her relationships. It can also highlight her determination. Even when hurt, she may still try to fight the bad guys or protect the innocent.