One plot idea could be that Supergirl and Batgirl team up to take on a new super - villain in Gotham. This villain has the ability to manipulate technology and is causing chaos. They combine Supergirl's super strength and flight with Batgirl's detective skills and martial arts expertise. Another idea is that they are transported to an alternate dimension where they have to find a way back home while facing different versions of their friends and foes. And a third idea could be that a powerful alien artifact is found in Metropolis and both girls must protect it from various forces that want to use it for evil, all while learning more about each other's worlds and abilities.
Perhaps a plot where Batman and Batgirl are investigating a series of strange disappearances in Gotham. They find out that it's related to a secret underground organization that is using some sort of new technology. Along the way, they start to realize their feelings for each other are deeper than just being crime - fighting partners. But they have to put their personal feelings aside to stop the organization before it's too late.
One idea could be that the OC is a new vigilante in Gotham and Batgirl takes them under her wing to train them. This could lead to a lot of action - packed scenes as they fight crime together and the OC learns from Batgirl's experience.
One idea could be that the male self - insert (SI) has a unique power that complements Supergirl's. For example, he might have the ability to control technology, and together they fight against a cyber - villain who is trying to take over the city's power grid. Another plot could involve the male SI coming from a different dimension and having to adjust to the world of Supergirl while also helping her deal with an otherworldly threat that she's not familiar with. A third plot could be that the male SI is an ordinary human at first but gains powers through an accident and then he and Supergirl have to learn to work together as he masters his new abilities.
A cool plot could be that there's a mysterious threat targeting all of Batman's allies. Robin and Batgirl team up to not only find out who's behind it but also to protect each other. Along the way, they share some personal stories from their pasts that bring them closer together.
One idea could be that Harry, with his magical skills from the wizarding world, somehow gets transported to Gotham City. He teams up with Batgirl to fight a new kind of villain who is using a combination of magic and technology. Harry could use spells like Protego to shield them from attacks, while Batgirl uses her martial arts and gadgets.
A plot could involve a time - travel scenario where Batman and Supergirl are sent back in time to prevent a major event that would change the future of both their worlds. They would have to work together, using their different skills and knowledge of their respective eras.
One idea could be a dark Supergirl who is corrupted by a powerful, evil force from outer space. She starts using her powers for her own gain, causing chaos in Metropolis. Another could be a Supergirl who has lost faith in humanity due to some great betrayal and turns to a darker path to 'teach' humans a lesson. And a third idea might be a Supergirl from an alternate, darker universe where her powers are more unstable and she has to constantly battle her inner demons.
Perhaps in the fanfiction, Supergirl is drawn to the Naruto world because of a strange energy signal similar to Kryptonite. Once there, she teams up with Sasuke who is initially distrustful but comes to respect her. They go on a quest to find the source of this energy. Along the way, Supergirl discovers that the energy is actually a by - product of an ancient and powerful jutsu. This jutsu was sealed away long ago, and now it's starting to leak, threatening both the Naruto world and potentially Supergirl's home planet if it spreads.
Another plot could be set in an alternate universe where Supergirl lands in a world that has no concept of superheroes. She has to learn how to fit in while also trying to figure out how to use her powers without causing chaos. The fanfiction could explore her journey of self - discovery in this new world.
One plot idea could be that Supergirl has a secret mission that Chrom accidentally stumbles upon. This leads to them having to work together, initially reluctantly, but then they discover they have complementary skills. For example, Supergirl's superpowers and Chrom's strategic thinking. As they face various challenges together, they form a strong bond.