One idea could be that the female vampire OC is initially a foe. She might be causing chaos in the city, perhaps by preying on criminals at first but then getting out of control. The Justice League has to figure out how to stop her without killing her. They discover she has a tragic backstory that led her to become a vampire, and they try to rehabilitate her.
The female vampire OC could be a time - traveling vampire. She comes from a future where the Justice League has failed and the world is in chaos. She travels back in time to join the current Justice League and change the course of events. Along the way, she has to deal with her own vampiric instincts while trying to fit in with the heroes and save the world.
Think about her powers and how they would be reflected in her design. If she has the ability to turn into a bat, she could have some bat - like features even in her human form, like small, pointed ears or a slightly more angular face. Her hair could be wild and unkempt, as if she's constantly on the move. And her shoes should be practical for running and sneaking around.
One idea could be an OC vampire who joins the Justice League under false pretenses. Maybe they claim to want to use their powers for good, but in reality, they have a hidden agenda related to their vampiric nature, like finding a powerful magical artifact within the Justice League's HQ.
Perhaps in the fanfiction, there is a parallel universe where the Justice League are all vampires. They are trying to protect both the vampire and human populations from a greater evil that threatens to destroy both. They face challenges in getting the human - dominated Justice League in our universe to trust them and work together.
One idea could be an OC with the power to manipulate time joining the Justice League. They could accidentally create alternate timelines while trying to stop a villain, and the Justice League has to fix it. For example, the OC might go back in time to prevent a major disaster but end up changing the course of events in a way that makes the villain even more powerful in the new timeline.
The OC shop owner could be a former member of a secret society that has knowledge about the Justice League's enemies. They use their shop as a cover while secretly helping the League by providing intel. For example, they overhear conversations from villains who visit the shop thinking it's just an ordinary place.
First, develop your OC vampire's character well. Decide on their backstory, like how they became a vampire and what their values are. For instance, if they were turned against their will, they might have a strong sense of justice to fight against other evil vampires.
One idea could be an Immortal OC who has ancient knowledge and uses it to help the Justice League prevent a threat that was foretold in a long - lost prophecy. For example, this OC might know of a hidden weakness in an upcoming powerful villain.
Maybe the OC Kryptonian has a power that is malfunctioning. They struggle to control it and it causes problems within the Justice League at first. But through the help of other members like Superman, they learn to master it. This could lead to a story of self - discovery and friendship within the team. They could face a villain who specifically targets their power, making it a race against time to get it under control.
One idea could be an OC who is a long - lost relative of Aquaman. They could have unique powers related to water control but in a different way than Aquaman. For example, the OC can turn water into different shapes and use them as weapons. Another plot could involve the OC being from a hidden underwater tribe that has a different view on Atlantis's rule. The OC could try to bridge the gap between their tribe and Atlantis while also joining the Justice League to fight common threats.
One idea could be that the OC has a unique power that the Justice League initially distrusts but later comes to rely on. For example, the OC can manipulate time in small bursts, which at first seems dangerous but is actually crucial in stopping a villain who can freeze time.