One great Tokyo Ghoul Kaneki crossover fanfiction is the one where he crosses over with the world of 'Attack on Titan'. It's really interesting to see how Kaneki's ghoul abilities interact with the Titans. The author does a great job of creating a believable scenario where the two worlds collide.
One interesting female Kaneki fanfiction could be one where she forms an unexpected alliance with a group of ghouls from a different territory. In this story, she might use her unique perspective as both human and ghoul to bridge the gap between different ghoul factions. It could explore her internal struggle in a new light as she tries to maintain her humanity while also fitting into the ghoul world.
One possible fanfiction could be where Kaneki, after fully mastering his ghoul powers, becomes a sort of protector in the ghoul world. He could use his strength to keep the balance between humans and ghouls, defying the traditional enmity. He might be able to create new rules that both sides can abide by, making the world a more harmonious place for both species.
In a fanfiction where Kaneki is overpowered, he could have the power to heal the rift between humans and ghouls in a more radical way. He could use his extreme strength to force the humans to accept the existence of ghouls and also make the ghouls understand the importance of not preying on humans. His overpowered abilities would be like a shockwave that shakes the very foundation of the existing relationships and rebuilds them in a more positive way. For instance, he could create a safe zone where both humans and ghouls can live together without fear, and his overpowered presence would ensure that no one breaks the peace.
If Kaneki turns into a baby in fanfiction, it could be a way to explore his character in a new light. Maybe he retains some of his memories from his previous form, but is unable to communicate them clearly. This could lead to a lot of misunderstandings and new adventures. Also, it could be a chance for the author to develop new relationships for Kaneki, as a baby he would be more vulnerable and dependent on others.
Kaneki and Touka could be in a situation where they have to team up with some former enemies to save the ghoul community from a new threat. This would test their ability to forgive and their leadership skills within the community.
In some crossovers, like the one with 'Bleach', a really creative plot twist is when Kaneki's kagune somehow gets linked to a Hollow's power. It completely changes the power dynamics and makes for some intense battles. The idea that his ghoul nature could be combined with Hollow abilities in such a way was unexpected and exciting.
Self - acceptance is a very prevalent theme. The two Kanekis need to come to terms with their own existence and their relationship with each other. Identity crisis is also common. They both represent different aspects of Kaneki's identity and the fanfic often explores how to reconcile these different identities. Additionally, the theme of power and control can be seen, as they may fight over who gets to be in control in certain situations.