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.
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.
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 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.
One idea could be to have Kaneki enter a parallel universe where he has to team up with a powerful centipede - like being to fight a common enemy. Maybe they are both trying to stop an evil organization that is experimenting on ghouls and creating mutant centipedes.
One possible plotline could be that Harry Potter somehow gets turned into a half - ghoul like Kaneki Ken. He might then have to struggle with his new identity while still dealing with the threats in the wizarding world. For example, he could be hunted by both Death Eaters and those in the wizarding world who fear his new ghoul - like nature, as well as face the challenges that Kaneki faces in Tokyo Ghoul like finding a place to belong.
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.