One common theme is teamwork. Given that all three characters are powerful in their own universes, fanfictions often explore how they work together to fight a common enemy. For example, they might team up against an alien threat that is too powerful for any one of them to handle alone.
Well, another place could be some dedicated superhero or Dragon Ball fan forums. Sometimes fans share their own fanfictions there. For example, the Dragon Ball Z subreddit might have threads related to Gohan crossovers, and you could potentially find links to fanfictions involving Starfire and Supergirl too. It might take a bit more digging though compared to the big fanfiction sites.
Supergirl has super strength which allows her to lift extremely heavy objects. She also has heat vision that can be used as a weapon. And of course, she can fly. Wonder Woman, on the other hand, has super strength as well, but she has unique weapons like the Lasso of Truth which can force people to tell the truth, and her indestructible bracelets which can deflect bullets.
Supergirl could potentially win. She has the same powers as Superman under a yellow sun. Her heat vision can be extremely powerful. If she uses her speed and heat vision in a coordinated way, she could overpower Wonder Woman. Also, Supergirl might be more adaptable in different environments as she has had to face various threats on Earth and in space.
The use of alternate universes is also popular. In an alternate universe, Supergirl could have different powers or a different personality, and the same goes for Doomsday. This allows for unique and unexpected storylines where the outcome of their battle is completely different from the mainstream comics or shows.
There are also fanfictions where the battle is a draw. Maybe they fight to a standstill, or some external factor intervenes, like another superhero coming in and stopping the fight before there's a clear winner. The beauty of fanfiction is that it can take different directions depending on the writer's creativity.
Supergirl is a popular character in fanfiction. Writers love to explore her powers, her dual identity, and her relationships. Her superhuman abilities make for exciting action scenes, and her kind heart makes her a beloved hero in many stories.
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.