A crossover with "The Avengers" would also be cool. Kaneki could team up with the Avengers to fight against a common enemy that threatens both the human and ghoul worlds. His unique abilities would add an interesting element to the superhero team.
Archive of Our Own (AO3) is a great place to start. It has a large collection of fanfiction from various fandoms, including many Ken Kaneki crossovers. You can use the search function to narrow down your options by specific crossovers or tags.
One plot idea could be that Ken Kaneki somehow gets transported to the world of 'Fairy Tail'. He might initially be seen as a threat due to his ghoul nature, but as the story progresses, he could form alliances with the Fairy Tail guild members. Maybe he helps them fight against a common enemy that has some similarities to the ghoul - like threats in his own world.
Sure. You can try searching on popular fanfiction platforms like Archive of Our Own (AO3). There are likely many works that cross over Ken Kaneki from 'Tokyo Ghoul' with the world of 'Fairy Tail'. Just type in the relevant keywords in the search bar and you'll be presented with a variety of stories.
One idea could be a crossover with the 'Harry Potter' universe. Ken Kaneki could be a new student at Hogwarts, dealing with his ghoul powers while also learning magic. Another could be a crossover with 'Attack on Titan', where he helps the Scouts fight Titans, using his unique abilities. Or perhaps a crossover with 'My Hero Academia', where he attends UA and has to navigate the world of heroes and villains.
You can try Archive of Our Own (AO3). It has a vast collection of fanfics, including many 'ken kaneki crossover' ones. Just use the search function and type in relevant keywords like 'ken kaneki crossover'. Another option is Fanfiction.net. It also has a good number of such fanfics, although the interface might be a bit older - fashioned.
One possible scenario in a Naruto - Kaneki fanfiction could be that Kaneki somehow ends up in the Naruto world. Maybe through a strange portal or some sort of jutsu gone wrong. He might initially struggle to adapt to the different environment and the presence of chakra. But his own unique kagune abilities could be seen as a strange new form of power in the Naruto universe.
A common theme is romance. Since Ken Kaneki is a complex and interesting character, many female - authored fanfictions explore a romantic relationship between a female character and him. Another theme is friendship. Female characters might form deep friendships with Ken Kaneki and support him through his struggles. Also, self - discovery is often there. Ken Kaneki's own journey of finding out who he is as a half - ghoul can be paralleled with a female character's journey in some fanfictions.
One interesting story could be about Kaneki Ken's unique abilities being introduced into the RWBY world and how he teams up with Team RWBY. Maybe his kagune could be a new form of combat power that the RWBY characters have never seen before.