A mysterious janitor who actually has a past connection to Koro - sensei. He could be secretly helping or hindering the students' efforts, and his presence could add an element of mystery and suspense to the fanfic.
A new transfer student from a foreign assassin school. This character could bring in different assassination techniques and a different cultural perspective on dealing with Koro - sensei. They might be more ruthless or have a unique code of ethics.
One idea could be a new student arriving at Paradise High with a special ability that both helps and complicates the assassination attempts on Koro - sensei. For example, they might be able to sense his weak points more acutely but also have a personal vendetta against the students trying to kill him.
Karma Akabane would also be a likely main character. His mischievous and intelligent nature would have evolved over the years. He could be involved in some sort of high - stakes international work, using his sharp mind and combat skills. And of course, he might still have that competitive edge when it comes to his former classmates.
Nagisa Shiota is also very popular. Fanfics could focus on his growth as an assassin and how his initially meek personality evolves over time. His unique skills and his friendship with other classmates can be great elements to build a story around.
The children of Nagisa and Kayano could be main characters. They might inherit some of their parents' skills and quirks. For example, Nagisa's son could have his father's stealthy nature.
There could be a new neighbor in Jenna's apartment building. A quirky artist who inspires Jenna in new ways, either in her personal life or her career at the magazine.
Nagisa would be a great character to pair with Okuda. Nagisa is very observant and could help Okuda in her experiments by noticing small details that she might miss. Also, their different personalities would create an interesting dynamic. Karma could also be a good pairing. He's very intelligent in his own way and his mischievous nature could either help or hinder Okuda's scientific pursuits, leading to some fun and tense situations.
Nagisa could be one. He has a kind heart and is very intelligent, which could attract the main character. Also, Karma's confident and somewhat rebellious nature might make him an interesting addition to the harem in a unique way. And Kayano, with her cheerful personality and hidden secrets, could also be a potential love interest.
Itona and Nagisa could be an interesting pairing. Nagisa's calm and understanding nature could balance out Itona's more impulsive side. They could have adventures together in the classroom, like working on a secret project to surprise the teacher.
In a 'henceforward rwby fanfic', a new character could be a young boy from a nomadic tribe in Remnant. His semblance is the ability to sense danger from miles away, which has kept his tribe safe for generations. He could join the RWBY gang on their adventures, bringing with him a different perspective on the world. Another potential character is a female knight from an ancient order in Atlas. She has a very strict code of honor and her semblance is the ability to make her armor impenetrable for a short period. Also, a character from a hidden underground city in Remnant could be introduced. This character has a semblance that allows them to phase through solid objects and could provide some interesting plot twists in the fanfic.
A new character could be a demigod who has the power to control time in small bursts. They could be from a really old and secretive family of demigods that no one knew about. This character could cause a lot of chaos or be a great help to Percy depending on their motives.