Another plot could involve the OC being a time - traveler who gets stuck in the world of 'knight and day'. They try to find a way back home while also getting involved in the main story. They could have future technology that the characters in the original story find both fascinating and dangerous.
The OC could be a shapeshifter who is initially a mystery to the knight and day characters. They use their shapeshifting abilities to deceive the villains at first but then have to reveal their true self to gain the trust of the heroes. They end up becoming an important part of the team, using their unique ability to solve various problems.
One idea could be that the OC is a long - lost relative of one of the main characters in 'knight and day'. They show up unexpectedly and have some special skills or knowledge that helps in the adventure. For example, they might be an expert in a certain type of martial art that is useful against the villains.
One idea could be an OC pureblood who has the power to control dreams. They could use this power to either protect the school from hidden threats or accidentally cause chaos by interfering with the dreams of the students. For example, the OC might enter Yuuki's dreams to show her hidden memories of her past that she doesn't remember.
One idea could be an OC who is a brilliant scientist that discovers a new weakness in the alien technology just in time for the Independence Day battle. Another could be an OC who is a pilot from a small town and has to step up and fight the aliens, bringing a sense of hometown heroism to the story. Maybe an OC who is a diplomat trying to negotiate with other countries during the alien invasion to unite the Earth's defenses more effectively.
For Vampire Knight Pendragon OC fanfiction, an OC could be a time - traveler who accidentally lands in the middle of the vampire - human conflict. This OC would have to use their knowledge of different eras to try and find a solution. Another concept is an OC who is a vampire - hunter but has a change of heart when they learn more about the vampires in the Vampire Knight Pendragon world. Maybe they fall in love with a vampire. And finally, an OC who is a lost heir to a powerful vampire family in the story could be an exciting idea. Their arrival could disrupt the existing power balance and create a whole new set of storylines.
Well, an interesting plot could be that the OC is a transfer student who is secretly a vampire hunter but falls in love with a vampire at the academy. It would create a great internal conflict for the character. Also, the OC could be a half - vampire, half - human who is trying to find her place in both worlds. Maybe she has an ability that neither vampires nor humans have, like the power to heal vampire bites on humans. This would make her a target for both sides who want to use her power for their own purposes.
One idea could be to create an OC who is a time - traveling medic. They could cross paths with the Doctor during the 'day of the doctor' and use their unique medical skills to help in a crisis. Maybe they have a device that can heal wounds in a different way than the Doctor's sonic screwdriver.
Perhaps a fanfiction could focus on the backstory of Harvey Dent before he became Two - Face. It could show his early days as a prosecutor and how his idealism gradually got corrupted, which would add more depth to his character in the Dark Knight universe.
Perhaps an OC is a young archaeology student who idolizes Nathan Drake. They manage to get involved in one of his expeditions through sheer determination. Along the way, they learn from Drake and also teach him new things about modern archaeological research methods. It could be a story of growth and adventure for both characters.
A more action - packed plot could be the red knight leading a rebellion against the vampire hierarchy. They gather other vampires and humans who are tired of the oppression and fight for a new order. Along the way, they discover secrets about the vampire world that could change everything.
One idea could be an OC who is a lightning - style user in Kumogakure and has to face a powerful enemy from another village. The OC could discover a new jutsu during the battle.
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.