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.
Sure. You can try looking on fanfic websites like Archive of Our Own. Just search for 'kaneki ken x yonebayashi' in their search bar and you'll likely find a variety of fanfics.
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.
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 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.
Characterization is key. The fanfic should accurately portray Kaneki and Touka's personalities as we know them from the original work. For example, Kaneki's internal struggle and Touka's strong - willed nature need to be present. Also, the lemon aspect should be well - integrated, not just added randomly. It should flow with the story's plot and the development of their relationship.
There's 'Unlikely Allies' which is a great 'kaneki ken x hinami fanfiction'. It delves into how they support each other during difficult times. The story is full of emotional moments and their interactions are very well - written.