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.
I really like 'Kaneki's Female Companion'. In this fanfic, the female character has a unique backstory that intertwines with Kaneki's. It shows how their relationship develops in the dark and dangerous world of Tokyo Ghoul. The author does a great job of keeping the essence of the original Tokyo Ghoul while adding their own creative twists.
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.
A fanfic could focus on Rize teaching Kaneki how to fully embrace his ghoul side. She could show him how to use his powers more effectively and how to hunt without attracting too much attention. In return, Kaneki could teach Rize about human emotions and values, which she might have lost touch with over the years of being a ghoul. This exchange could lead to a deep bond between them.
A fanfiction might explore the idea of Kaneki and Touka's married life in a more comical way. Maybe they argue about who does the dishes, as ghouls don't really have normal eating habits. Or they have to deal with their friends constantly dropping by unannounced. Their relationship would still be full of love, but these little everyday situations would add a touch of humor to their married life.
One possible fanfiction could focus on Kaneki and Touka building a life together after the chaos in Tokyo. They might start a small coffee shop where they can interact with both ghouls and humans in a more peaceful way. This could be a symbol of their hope for a harmonious co - existence between the two species.