A theme of forgiveness might be present. Harry might have to forgive Narcissa for her past actions or associations, and in turn, Narcissa could forgive the wizarding world for how it has treated her family in some ways. This theme can lead to character development for both of them and create a more complex and engaging story.
One common theme could be redemption. Narcissa might be on a path of redeeming herself through her interactions with Harry. Another theme could be the exploration of family ties. Since Harry has a complex relationship with his own family and Narcissa has her own family issues, this could be a central aspect. For example, they might discuss how family doesn't always mean blood, and how they can create new family - like bonds with people outside their traditional families.
Friendship is also a likely theme. They could start off as unlikely allies and grow into friends. In these fanfics, they might share secrets, support each other through difficult times, and learn from one another. Harry could teach Narcissa about the value of standing up for what's right regardless of blood status, and Narcissa could teach Harry about the hidden corners of the wizarding world that only someone from her background would know.