Batman and Robin is a classic pairing. It's not just a superhero - sidekick relationship but also has a deep emotional bond. Their crossovers often explore Robin's growth under Batman's tutelage and how they handle different threats together.
A cool plot could involve Wonder Woman, Flash, and Aquaman. They find themselves in an alternate dimension where the rules of their powers are all mixed up. Flash might not be as fast, Aquaman can't communicate with sea creatures as well, and Wonder Woman's lasso doesn't work as expected. They have to figure out how to adapt and work together to get back to their own dimension while also facing the threats in this new world.
How about a crossover with the Gotham City Sirens? Harley Quinn, Poison Ivy, and Catwoman from that group could interact with the DC Superhero Girls. There could be a lot of fun and chaos as the girls try to either reform or fight against the Sirens. Maybe they end up having to work together when a bigger threat to Gotham emerges that affects both the good and the bad guys.
Supergirl and Bumblebee could be a great pairing. Supergirl's super strength and flight combined with Bumblebee's ability to shrink and fly as well could lead to some interesting scenarios. They could infiltrate small spaces together or Supergirl could carry Bumblebee to quickly reach far - off locations.
Definitely Wonder Woman. She's such an iconic character and in crossovers, she can bring her strength and wisdom. For example, if she crosses over with the Birds of Prey, she could lead the team in a fight against some new threat to the city.
Maybe a crossover with the Justice League. The DC Superhero Girls look up to the League, and in this fanfiction, they could be called upon to help when the League is incapacitated by a powerful magic spell. It would be a chance for the girls to prove themselves on a bigger stage.
One popular 'dc superhero girls fanfic' story could be about Supergirl's adventures in a new dimension. Another might focus on Wonder Woman training the younger superhero girls. And there could also be stories where Batgirl teams up with other heroines to solve a mystery in Gotham City.
You can try fanfiction.net. It has a wide range of 'dc superhero girls fanfic' stories. There are many different genres and styles to choose from there.