Well, an interesting Watchmen OC fanfiction could center around an OC who is a former sidekick of one of the main heroes. This OC could be struggling with the legacy left behind by their mentor. They might have a different moral code compared to the other characters in the Watchmen universe. Also, an OC who is an artist that uses their work to comment on the state of society in the Watchmen world, like Rorschach's journal but in a more public and creative way.
For Watchmen OC fanfiction, consider an OC who is an immigrant in the Watchmen universe. They bring their own cultural beliefs and values, which clash or blend with the existing society. This OC could be caught in the middle of the power struggles between the different factions. Or an OC who is a hacker, able to access and manipulate the information networks in the Watchmen world, uncovering hidden agendas and secrets that the other characters are unaware of.
One idea could be exploring what would happen if Dr. Manhattan had a different origin. Maybe he was a scientist working on a different project that still led to his transformation. Another could be a story set in an alternate universe where the Comedian never died and how that would change the whole Watchmen universe. And a third idea is a fanfiction focusing on Laurie Jupiter's life before she became Silk Spectre, her training and her family dynamics.
One interesting plot idea could be that a character from DC, like Batman, somehow ends up in the Watchmen universe and has to deal with the moral ambiguity there. He might find himself clashing with Rorschach over their different views on justice.
One idea could be creating an OC who has a unique power related to time manipulation. They could be from a different dimension and end up in the Avengers' world. Another idea is an OC who is a master of ancient magic and teams up with Doctor Strange within the Avengers group. Maybe an OC that is a super - intelligent android created by a villain but has a change of heart and joins the Avengers.
You could create an OC for your Persona OC fanfiction who is an artist. This artist OC could use their art as a way to understand and even manipulate the Metaverse. For example, they might draw scenes from the Metaverse that give clues about upcoming events or hidden locations. Their art could also be a form of therapy for other characters who are struggling with their dual lives in the real world and the Metaverse.
Perhaps an OC who is a half - human, half - beast creature. This OC could struggle with acceptance in both the human and beast - kin societies. They end up meeting the main characters during their adventures and learn to embrace their unique identity with the help of the group, while also adding their own combat skills, which are a blend of human strategy and beast - like strength, to the team.
One idea could be to explore the Riddler's origin story from the perspective of your original character (OC). Maybe your OC is a detective trying to solve the mystery of the Riddler's first crime spree. Another idea is to have your OC be a sidekick of the Riddler, either willingly or by being forced into it. They could then have a complex relationship where the OC tries to influence the Riddler's actions for better or worse.
One idea could be an OC who is a brilliant scientist in the Fallout world. They could be working on a secret project to reverse the effects of radiation. Another idea is an OC who is a wandering trader, having all kinds of adventures while traveling from one settlement to another, facing raiders and mutants along the way. And a third idea could be an OC that is part of a new faction trying to bring peace and order to the wasteland.
You could also create an OC human who discovers a hidden talent for communicating with trolls through music. In the fanfiction, this character could use their musical ability to bridge the gap between the two worlds, maybe even creating a new form of harmony between trolls and humans that was previously unthinkable.
One idea could be to create a story where your Bratz OC is a new student in Stilesville. She has a unique fashion sense that both confuses and intrigues the other Bratz girls at first. As the story progresses, they become friends and she helps them design a whole new line of clothing for an upcoming fashion show.
One idea could be creating an OC who is a transfer student with a unique past related to the Dark Hour. For example, they might have had a family member affected by Shadows long before coming to the school. Another idea is an OC with a special ability that doesn't quite fit the normal Persona - using skills that are more about mental manipulation rather than direct combat. And you could also have an OC who is initially a skeptic about the whole Persona thing but slowly gets drawn into it.