If it's an illness, they could search for a cure. Maybe there's a special herb or a magic spell in the world of the fanfic that can heal her. Her friends could go on a quest to find this cure, and in the process, there could be more character development and interesting plot twists.
She might be a character representing the general state of women in a particular society or situation. Maybe she's a symbol for all those who are oppressed and are finally finding their voice to scream out in protest.
Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.