You could create an OC who is a ghoul investigator from a foreign country. This OC would bring different techniques and perspectives to the fight against ghouls. They might also face discrimination or misunderstanding within the Tokyo Ghoul universe due to being an outsider. For example, they could have a special weapon or training method that the local investigators haven't seen before.
One plot idea could be your OC being a new ghoul trying to find their place in the ghoul society. They might start at the bottom, having to learn the rules of the different ghoul groups and facing discrimination from more established ghouls. Another idea is that your OC discovers a secret about their transformation that no one else knows, like it was part of a secret experiment by the CCG. They then have to figure out what to do with this knowledge while also avoiding being caught by the CCG.
One idea could be a fanfic where a human character tries to form a peaceful co - existence pact with the ghouls. They might start by secretly helping a friendly ghoul and gradually work towards getting both sides to understand each other better.
Some plot ideas for Tokyo Ghoul female OC fanfiction include her being an outcast among ghouls due to her different beliefs. Maybe she believes in non - violence and tries to spread this message, which goes against the traditional ways of many ghouls. Or, she could be on a quest to find out the truth about her transformation into a ghoul. Was it an accident, or was she targeted? This could lead her to uncover secrets about the ghoul world that have been long buried.
One plot idea could be that the normal human OC discovers a secret organization that is trying to create a peace treaty between ghouls and humans. The OC gets involved in the negotiations, facing opposition from both sides. Another idea is that the OC is a detective who is investigating a series of ghoul - related murders and ends up getting too close to the truth and becomes a target. Or perhaps the OC is a doctor who tries to find a way to make ghouls live without having to eat humans.
Another aspect could be his relationships. He might have a complicated friendship with a human who is aware of his ghoul nature. This relationship could be full of tension, as they both try to navigate the dangerous world where ghouls and humans are at odds.
A good plot could involve a human OC who is a doctor. They end up treating a ghoul who was severely injured in a fight. This starts a strange relationship where they try to understand the nature of ghouls better. As they get more involved, they learn about a secret organization within the ghoul world that is planning something big that could endanger all of Tokyo. The OC then has to decide whether to stay out of it or try to stop it. It's a plot full of moral dilemmas and action. Also, a plot where the human OC is an artist and they use their art to show the hidden side of the ghoul - human relationship could be interesting. Their art could either be a way to promote peace or it could stir up more trouble depending on how different groups react to it.
One possible plot could be that the oc is reborn as a ghoul in the Tokyo Ghoul world. They struggle to come to terms with their new identity and the need to feed on human flesh. They could try to find a way to co - exist with humans without causing harm, perhaps by joining Anteiku and learning from Kaneki and the others.
Maybe a story where a Tokyo Ghoul character gets transferred to the My Hero Academia universe. They could be mistaken for a villain at first because of their ghoul appearance, but then prove themselves to be an ally. For instance, Kaneki could use his kagune in unique ways during hero training exercises, impressing the teachers and students alike.
One plot idea could be that a character from Tokyo Ghoul ends up in Ikebukuro (the setting of Durarara). For example, Kaneki could find himself in the middle of the Dollars' activities. He might initially be confused by the different kind of chaos compared to the ghoul world. Another idea is that Celty's head mystery somehow gets intertwined with the ghouls' existence. Maybe there's a ghoul who has knowledge about her head, and this leads to an alliance between Celty and some of the Tokyo Ghoul characters to find out the truth. A third plot could involve the gangs in Durarara getting involved with the ghoul - human conflict in Tokyo Ghoul. They might try to take advantage of the situation for their own gains, but end up getting more than they bargained for as they learn about the true nature of ghouls.
One idea could be exploring the Black Reaper's inner turmoil as he struggles between his human and ghoul sides more deeply. Maybe he has to make a choice between protecting a group of innocent humans or siding with a ghoul community that has shown him kindness. Another could be a time - travel plot where the Black Reaper goes back to a crucial moment in his past to change something, but discovers that changing the past has unforeseen and dangerous consequences. And a third idea could be a story where the Black Reaper teams up with an unlikely ally, like a human detective who is also trying to solve a mystery related to the ghoul underworld.