You could also focus on the relationship between different courts in the 'a court of mist and fury' world. Maybe create a story where two courts that are usually at odds have to work together against a common enemy, and show how the characters from different courts interact and grow through this experience.
A great plot could be a 'what if' scenario. What if the main character made a different choice at a crucial moment? Like if Feyre didn't go with Rhysand at a certain point. This could lead to a completely different set of events and relationships in the fanfiction.