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.
Tatsumi from 'Akame ga Kill!' would be a great pair. Tatsumi is a fighter who has a strong sense of justice, and his interaction with Kaneki, who has his own moral struggles as a ghoul, could create an interesting dynamic. They could fight together against common enemies in a crossover world.
One idea could be a crossover with the world of 'Fullmetal Alchemist'. Kaneki Ken could be a chimera created by some dark alchemical experiment. His kagune could be seen as a strange alchemical creation. Another could be a crossover with 'Attack on Titan', where Kaneki joins the Survey Corps and uses his kagune to fight Titans in a unique way.
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 common theme is the exploration of a different power dynamic. Since Kaneki is male in the original, making her female changes how she might interact with other characters in terms of power struggles. Another theme could be the re - imagining of her relationships, like with Touka. Maybe a female Kaneki Ken would have a more nuanced friendship or romance with Touka.
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.
There might be a fanfic called 'Touka and Kaneki: Lemon Love'. In this story, it might explore how their love blossoms in a more intimate and passionate way, while still staying true to their original personalities from the source material. It could include tender moments and some steamy scenes that are tastefully written.
One common theme could be the exploration of their relationship dynamics. Since they are both characters from Tokyo Ghoul, their different personalities and experiences within the Tokyo Ghoul universe might be explored.
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.