You could also have a male OC who is a prodigy in a non - assassination related field, like art or music, but is forced to be in the assassination classroom. His struggle between his passion for his art and his duty to assassinate Koro - sensei could be a central theme. He might find unique ways to use his artistic skills in the assassination attempts, like creating a musical distraction or using his painting skills to camouflage himself.
First, give your male OC a clear backstory. For instance, if he has a tragic past, like losing his family to an assassin, it can explain his motivation to be in the assassination classroom. Second, define his unique skills. It could be something physical like superhuman speed or a mental ability like mind - reading. This will make him stand out among the other characters.
One idea could be an OC who has a unique ability related to stealth. For example, they can blend into the shadows like a chameleon. This OC could use this ability to get closer to Koro - sensei without being detected, adding a new dynamic to the assassination attempts. Another plot could involve an OC who is a transfer student from a different school with a completely different teaching style. They struggle to adapt to the chaos of Class 3 - E at first but eventually find their place and contribute to the assassination plans in their own way. Maybe an OC who is a master of disguise and uses it to trick Koro - sensei into various situations where they can try to attack him.
First, think about their backstory. Where did they come from? Were they bullied in their previous school? Or did they have a family tragedy that led them to Class 3 - E? This will shape their personality. For example, if they were bullied, they might be more reserved but also determined to prove themselves. Second, give them unique skills or talents. It doesn't have to be combat - related. It could be something like being really good at cooking, which can be used in a creative way in the story. Third, consider their relationships with other characters. Do they admire Nagisa? Or do they have a friendly rivalry with Karma?
One possible fanfiction could be about Nagisa, as a girl, having to deal with the unique challenges in the Assassination Classroom. Maybe she uses her newfound femininity to her advantage in different assassination attempts. For example, she could play on the stereotypes that others might have of a girl being less of a threat, only to surprise her opponents.
One possible fanfiction could be about female Karma's journey in the Assassination Classroom. Maybe she uses her intelligence and unique skills to outwit the other students in various assassination attempts. For example, she could come up with creative plans to get close to Koro - sensei while still maintaining her cool and mysterious demeanor.
One characteristic is the exploration of the unique characters. Fanfictions often dig deeper into the backstories and personalities of the students and the teacher in 'Assassination Classroom'. For example, they might expand on how Nagisa came to be so calm and observant. Another aspect is the different plot twists. Some fanfictions create alternate scenarios where the assassination attempts take on new forms or have different outcomes.