One idea could be exploring Prisoner Zero's backstory before being imprisoned. Maybe he was wrongly accused and the oc in the fanfic could be on a quest to prove his innocence.
You could include a wise old inmate who has been in the prison for a long time and has a lot of knowledge about its secrets. This character could help the oc and Prisoner Zero in their plans.
One idea could be that the OC is a new student at the academy where Tabitha is. They accidentally cross paths during a magic experiment gone wrong. Tabitha initially sees the OC as a nuisance but gradually discovers they have a unique magical ability that could help her in her own goals. For example, the OC might be able to communicate with magical creatures that are crucial to a mystery Tabitha is trying to solve.
One idea could be that the patient zero OC has a unique ability to heal others despite being the origin of a disease. This creates a moral dilemma within the story as people are both afraid and in need of their power.
Another plot idea could be focusing on the backstories of the secondary characters in 'zero no tsukaima' within the fanfic. For example, we could learn more about Siesta's life before she came to the academy. It could add more depth to the overall story world and create new sub - plots.
A mystery - themed 'the oc fanfic' would also be great. Maybe there's a mystery surrounding the Cohen family's wealth or a long - lost relative that suddenly shows up with a hidden agenda. The characters would have to work together to solve the mystery, and in the process, their relationships would be tested and new alliances formed.
One idea could be exploring an OC's (Original Character) journey to become a leader within the 100's post - apocalyptic world. They could start as an outcast and through their unique skills, like advanced medical knowledge or exceptional hunting abilities, gain the respect of different factions.
One idea could be a male OC who is a powerful mage from a far - off land arriving at the Tristain Academy. He could have unique magic that no one has seen before, like the ability to control time on a small scale. This would create interesting interactions with the main characters, especially Louise. Another plot could involve the male OC being a spy from a rival country, but gradually falling in love with the world of Halkeginia and having to choose between his loyalty to his homeland and his new - found friends.
Another plot could be that the OC is an apprentice of an enemy clan. Sub Zero is sent to spy on the clan and meets the OC. The OC is different from the others in the clan, being more kind - hearted. Sub Zero sees this and decides to protect the OC from his own clan's wrath, leading to a complex relationship.
One idea could be exploring Sirius Black's time on the run in more detail. Maybe he had some secret encounters with other magical creatures that he didn't tell Harry about. Another could be a story from the perspective of Remus Lupin during his time as a teacher at Hogwarts that year, and how he was constantly worried about his werewolf secret being exposed while also trying to protect Harry. A third idea could be a fanfic that focuses on Hermione's time - turning device, and what would happen if she accidentally sent herself back too far in time and had to find a way to get back to the right time without disrupting the timeline too much.
One idea could be to cross it with a modern - day world. Imagine Louise and Saito suddenly finding themselves in a bustling city. They would have to deal with technology they don't understand, like smartphones and cars. Another plot could involve a crossover with a magical school from another series. The characters could exchange magical knowledge and spells.