One possible aspect of the 'kaneki and hide lemon fanfiction' could be the exploration of their relationship in a more romantic or intense way. Lemon often implies a certain level of sensuality in fanfiction, but it could also just be about deepening their emotional bond.
One common theme is identity. Since it's a female version of Kaneki Ken, the fanfiction often explores how her gender affects her identity as a ghoul. Another theme is relationships. There are different takes on how she interacts with other characters like Touka or Hide.
One popular 'kaneki ken fanfiction' story could be those that explore his transformation in more depth. For example, some might focus on his mental state during the different stages of becoming a ghoul. Another could be about his relationships with other characters like Touka, showing their interactions in new and different scenarios that weren't fully explored in the original work.
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.
It's likely that in this fanfiction, the author wants to take Kaneki Ken's character and put a new spin on it by making him a baby. Maybe it's a way to show how his character might have developed from such an early stage. It could also involve other characters from the series interacting with baby Kaneki in interesting ways, like how they might protect or care for him.
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.
I would recommend 'A New Beginning for Kaneki and Hide' which can be found on Archive of Our Own. It has a unique take on their relationship, exploring what could happen if Hide had a more active role in Kaneki's transformation journey. It's full of emotional moments and great dialogue.