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 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.
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.
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 common theme is the 'fish - out - of - water' story, like when an OC from a normal life suddenly gets thrown into the world of the Justice League. It creates a lot of opportunities for character growth and comedic moments as they struggle to understand the superhero world.
One great 'OC Justice League Fanfiction' is 'The New Hero Among Them'. It's about an original character with unique powers who joins the Justice League. The story focuses on how the OC adapts to the big - league environment and forms relationships with the well - known heroes. It has exciting action scenes where the OC's powers are put to good use against classic villains.
The OC's personality can be very distinct. They could be extremely humorous and use that to lighten the mood during tense situations. This would be different from the more serious natures of some of the Justice League members like Batman. Also, they could have a strong moral code that is not exactly the same as the League's, which would lead to interesting discussions and interactions.
One common theme is the struggle with identity. The Justice League members are used to being heroes in a certain way, and when they become vampires, they have to redefine who they are. Another theme is power and its abuse. Vampirism often brings new powers, and the fanfictions explore how the characters handle this power.
Often in such fanfiction, the Oc related to Old Gods can also be a vehicle for exploring different mythologies. The Justice League already deals with various superpowers and concepts, but the addition of Old Gods and an Oc associated with them can bring in elements from different cultural mythologies. This can make for a rich and diverse story, with the Oc acting as a bridge between the known superhero world and the more mysterious realm of the Old Gods.
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.