Well, a plot could involve Supergirl being from an alternate dimension where the events of Injustice 2 didn't occur as they did in the main universe. She arrives in the Injustice 2 world and Batman initially suspects her of being a threat. But as they interact, they find common ground in their desire to restore justice. They could team up to expose the true villains behind the chaos in the Injustice 2 world.
One idea could be that Batman tries to train Supergirl to better control her powers in the context of the Injustice 2 universe. He might see her as a potential force for good against the tyrannical regime. Another could be that Supergirl discovers a way to bring back the more heroic Superman she once knew, and Batman has to decide whether to trust her plan or not.
In a fanfiction, perhaps Supergirl is captured by the forces that oppose Batman. Batman then has to mount a rescue mission. During the rescue, they learn more about each other's motives and beliefs. Supergirl realizes that Batman's methods, though different from what she's used to, are necessary in this dark world. And Batman sees in Supergirl a symbol of hope that could inspire the people to rise against the unjust regime. This could lead to them forming a strong alliance to fight for a better future in the Injustice 2 setting.
One idea could be exploring Batman's internal struggle in the Injustice universe more deeply. Maybe he has a nightmare where he becomes like the tyrannical Superman, and this makes him question his own actions and methods even more.
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 exploring an alternate universe where Terry McGinnis (Batman Beyond) is pulled into the Injustice universe and has to deal with the morally compromised heroes. He might try to find a way to bring back the true sense of justice among them. For example, he could start by trying to reason with Superman, showing him the error of his ways through his own experiences in his own universe.
One idea could be that Supergirl and Batman team up to stop a powerful alien weapon from being used on Earth. Maybe the weapon was stolen from a Kryptonian ship and Batman uses his detective skills to find clues while Supergirl uses her superpowers to fight off the bad guys who have the weapon.
One idea could be that Batman and Wonder Woman have a major disagreement over how to deal with the Injustice regime. Batman wants to use more stealth and subterfuge while Wonder Woman believes in a more direct approach. This could lead to some intense battles between them, not physical but more of a battle of wits and ideologies.
One plot idea could be Batman trying to convince Wonder Woman to see the error of her ways in the Injustice universe. Maybe he shows her the long - term consequences of their actions on the common people. Another could be a story where Wonder Woman and Batman have to team up against a greater threat that makes them re - evaluate their stances in the Injustice conflict.
One idea could be that Batman and Supergirl team up to stop a powerful alien threat that is beyond the capabilities of either of them alone. They start off distrusting each other due to their different methods, but as they face challenges, they learn to work together and respect one another's skills.
Their relationship could develop through mutual respect. Batman respects Supergirl's power, and Supergirl respects Batman's intelligence and strategic skills. They start by working on small missions together and gradually build trust.
A story about an OC who is a healer in the Injustice 2 world could be really engaging. In a world full of battles and destruction, this OC could be trying to heal not only the physical wounds but also the emotional and psychological damage that the characters have endured. They might face challenges like being kidnapped by villains who want to use their healing powers for their own evil purposes.
One idea could be the Avengers facing off against a new, unjust system within their own superhero community. Maybe some heroes are being wrongly persecuted due to false accusations of using their powers for personal gain. This would create a lot of internal conflict as they try to clear their names and fight against the system.