In such a fanfiction, perhaps there's a development of an unexpected friendship. Despite the initial slave - master relationship, they could start to understand each other better. For example, Hermione might teach Draco about the value of kindness and equality through her actions, which gradually changes his view of her and others.
Well, one possible plot point could be the power dynamic exploration. It might show how Hermione copes with being in a subservient role to Draco and how she tries to subvert it. Maybe she uses her intelligence to outwit Draco in certain situations while still being in the 'slave' position.
Another aspect could be the reaction of their friends. Harry and Ron might be horrified to find out Hermione's situation and try to rescue her. On the other side, Draco's Slytherin friends could either support or oppose his treatment of Hermione, which could add a lot of drama and conflict to the story.