One potential theme is the discovery of self. Yashiro and Satoru might find new aspects of themselves as they become parents. For example, they might realize they have a stronger sense of protection or a deeper well of patience than they thought. The theme of change is also likely. Their lives are completely altered with the arrival of the child. Their daily routines, their goals, and even their relationships with others might change. And there could be a theme of hope. The child represents a new beginning, a hope for the future, and they are building a family full of possibilities.
A theme could be adaptation. Yashiro and Satoru have to adapt to their new roles as parents. This includes adapting to sleepless nights, changing diapers, and dealing with a crying baby. Another theme is unity. They need to be united in their parenting decisions and support each other. The fanfic could also explore the theme of inheritance. The child may inherit certain qualities or powers (if applicable in the original context) from Yashiro and Satoru, and how these inheritances shape the child's life and the family's future.