Well, it could start with a strange portal opening up that transports some of the Tokyo Ghoul characters into the Devilman's world or vice versa. At first, there would be confusion and mistrust. But as they start to understand each other's situations, they realize that they need to work together. The story could then explore how their different powers interact. Like, how would the CCG's quinque weapons fare against Devilman's foes? And how would the Devilman's allies react to the existence of ghouls?
Rize from Tokyo Ghoul and Sirene from Devilman would be an interesting pair. Rize is a very powerful and somewhat unpredictable ghoul, and Sirene is a demon with her own motives. They could either be at odds, constantly trying to outwit each other, or they could find common ground in their pursuit of power. Their interactions could be full of tension and mind games, which would make for an exciting part of the fanfiction.
One possible plot could be a character from Tokyo Ghoul ending up in the world of Vampire Knight. Maybe Kaneki, with his unique ghoul powers, could interact with the vampires and hunters in Vampire Knight. He might initially be seen as a threat, but as the story progresses, they could find common ground in their struggle for survival in a world that fears them.
The plot could involve a shared threat that forces the Tokyo Ghoul and Parasyte universes to collide. Maybe a new type of organism emerges that is even more dangerous than ghouls or parasites. The characters would then have to combine their unique abilities to combat this threat. For instance, Kaneki's kagune and Shinichi's enhanced physical abilities with his parasite Migi could be used in tandem. This would create an interesting dynamic as they try to figure out how to work together while also dealing with their own internal struggles related to their inhuman natures.
The plot might focus on the struggle for identity that both Kaneki and Shinichi face. Kaneki is caught between his human and ghoul sides, while Shinichi has to deal with having a parasite in him. In the crossover fanfic, they could exchange their experiences and learn from each other. They might also encounter a new type of being that combines the traits of ghouls and parasites, which forces them to re - evaluate their own identities and how they fit into this new world of the crossover.
The plot could involve the ghouls from Tokyo Ghoul being brought into the Avengers' world. Maybe the Avengers initially mistake the ghouls for new, dangerous threats. But as they learn more about the ghouls' situation, they might try to find a way to co - exist. For example, Kaneki could share his story with the Avengers, and they could be sympathetic. Then they could team up to fight a common enemy, like a new type of super - villain who wants to use the power of ghouls for evil purposes.
One possible plot could be that Uta, with his unique mask - making skills from Tokyo Ghoul, gets involved in a situation where he has to create a special mask for a character from the Uta no Prince - sama world. Maybe this mask gives the Uta no Prince - sama character some special powers related to the ghoul world, like enhanced hearing or the ability to see in the dark. And then they have to navigate through both the music - filled world of Uta no Prince - sama and the dangerous world of Tokyo Ghoul together.
A plot might involve a magical accident that transports some of the Tokyo Ghoul characters into the wizarding world. Kaneki could end up at Hogwarts and be sorted into a house. He'd struggle to hide his ghoul identity while also trying to find a way back home. At the same time, the wizards are intrigued by his unique abilities and some, like the more power - hungry ones, might try to use him for their own gain. Harry Potter and his friends would then have to decide whether to trust Kaneki or not.
One possible plot could be that Naruto and his friends are sent to the Tokyo Ghoul universe by some sort of space - time jutsu gone wrong. Naruto, with his strong sense of justice, would be immediately intrigued by the situation of the ghouls. He might try to find a way to co - exist between humans and ghouls, using his ninja way. He could team up with some of the more sympathetic ghouls, like Kaneki, and together they fight against the more extreme elements on both sides, whether it's the violent ghouls who only see humans as food or the humans who want to exterminate all ghouls without question.
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.
The storyline could be really interesting. Maybe the ghouls could be a new kind of dark creature in the Harry Potter world. The characters from Tokyo Ghoul, like Ken Kaneki, might attend Hogwarts. He could struggle with his ghoul nature while trying to fit in with the wizards and witches. The magic in the Harry Potter universe could also have an impact on the ghouls' abilities, perhaps suppressing or enhancing them.