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.
If we consider other characters, like Touka from Tokyo Ghoul, her combat skills combined with a parasite - like ability could be really interesting. Imagine if she had some of the regenerative abilities of a parasite. She could take more risks in battles. And for the parasites in Parasyte, if they could somehow access the kagune - like substances from ghouls, it might give them an entirely new form of attack or defense. They could potentially use it to create barriers or projectiles similar to how ghouls use their kagune.
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.
Kaneki's kagune could be used in combination with Shinichi's parasite - enhanced physical abilities. For example, Kaneki could use his kagune to create a distraction while Shinichi uses his speed and strength to attack the enemy. Or they could work together in a more direct way, with Shinichi using his parasite - hand to hold an enemy in place while Kaneki delivers a powerful kagune strike.
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.
Well, it could be about Jedi or Sith characters finding themselves in the Tokyo Ghoul universe. Maybe a Jedi has to use the Force to deal with the ghouls' kagune. They might try to bring peace between humans and ghouls, like they do in their own galaxy with different species.
Well, in a Tokyo Ghoul and Harry Potter crossover fanfiction, there could be a character who is both a ghoul and a wizard. This character would have to struggle with the dual nature of their existence. On one hand, they have the magic of the wizarding world, but on the other, they have the hunger for human flesh as a ghoul. They might use magic to try and control their ghoul instincts, like using a spell to suppress the urge to attack people.
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?
A 'Black Butler' and 'Tokyo Ghoul' crossover fanfiction could have Sebastian from 'Black Butler' serving CCG investigators in 'Tokyo Ghoul'. He could use his demonic powers to hunt down ghouls in a very stylish and efficient way, much like how he serves Ciel in his own series. His loyalty to the investigators would be a fascinating aspect to explore, as well as how the investigators react to his otherworldly abilities.