Another concept is a crossover of the powers. In the fanfic, Weiss could have some sort of ice - based kagune, which combines her Schnee family glyph powers with the unique abilities of a Tokyo Ghoul. Blake could use her shadow powers in a way that is similar to how ghouls move in the shadows, making her a more stealthy fighter in the Tokyo Ghoul setting.
A more complex plot could involve the Grimm from RWBY being related to the ghouls in some way. Maybe there was an ancient experiment that created both species, and now the characters from RWBY and Tokyo Ghoul have to work together to find out the truth behind their origins and stop a greater evil that threatens both of their worlds.
One idea could be a crossover where a character from Harry Potter accidentally travels to the world of Tokyo Ghoul and Seikirei. They might use their magic to help in the battles against the ghouls in Tokyo Ghoul and get involved with the spirit battles in Seikirei. For example, Harry Potter himself could arrive and use spells like 'Stupefy' to stun the ghouls or use magic to protect the Sekirei from harm.
One idea could be a crossover where a vampire from the Harry Potter universe somehow ends up in the world of Tokyo Ghoul. Maybe a vampire who was hiding in the Forbidden Forest gets transported to Tokyo and has to adapt to the new rules of the Tokyo Ghoul world, dealing with ghouls and the CCG. This could lead to some interesting power dynamics as the vampire's magic might be a new factor in the Tokyo Ghoul ecosystem.
First, do your research on both universes. Know the characters, their powers, and the settings well. For RWBY, understand the different academies, the concept of Dust, and the relationships between the main characters. For Tokyo Ghoul, learn about the CCG, the different types of ghouls, and the Anteiku coffee shop.
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.
It could be really interesting. Maybe the wizards in Harry Potter world have to face the ghouls from Tokyo Ghoul. The magic spells could be used to fight against the kagune of the ghouls. For example, the Stunning Spell might stun a ghoul temporarily, giving the wizard a chance to escape or attack further.
It could be a wild and unique story. Imagine the magic of Harry Potter in the same world as the ghouls from Tokyo Ghoul and the devilish elements of High School DxD. Maybe the characters from each would form unexpected alliances. For example, a Hogwarts student could team up with a ghoul to fight against the devils from High School DxD in a battle for some sort of magical artifact. Or perhaps the different power systems would interact in strange ways, like a wizard using spells to counter a ghoul's kagune while also fending off a devil's power attack.
One idea could be to explore the backstory of Rize's brother. Maybe he was involved in some secret ghoul society before Rize became known. You could start with a scene where he is on a mission for this society, which then ties into how Rize's life is affected.
One idea could be to focus on the male OC's unique kagune. Maybe it has a special ability that sets him apart from other ghouls in the Tokyo Ghoul universe. For example, it could be a kagune that can change its shape based on the situation, like becoming a shield in combat or a tool for climbing buildings quickly. Another aspect could be his backstory. He could be a half - ghoul, half - human, which would bring a whole new set of internal and external conflicts. He might struggle with his identity, not fully belonging to either the ghoul or human world. This could lead to interesting interactions with both ghouls and humans. Also, consider his relationships. He could have a mentor, perhaps an older ghoul who teaches him the ways of survival in the harsh ghoul society, or he could form an unlikely friendship with a human who doesn't know his true nature at first.
One popular crossover idea could be with 'Attack on Titan'. Picture Tokyo Ghoul characters in the world of Titans, fighting alongside or against the Survey Corps. Another could be with 'Fullmetal Alchemist', where the alchemical concepts might interact with the ghoul powers. For example, an alchemist trying to find a way to cure the ghouls or control their kagune.
Yes, it is quite common. Fans are always eager to explore more of the world they love, and fanfiction provides an opportunity to do that.