If you have another suggestions you can always comment and sorry for my English by the way I'm Dutch but I will try my best.
these are the themes I could think of to write about I can also add G!P,BDSM or both to the story
Themes for SWANQUEEN fanfic:
Boss/employer
Teacher/student
Teacher/teacher
Mother/teacher
BDSM
Social media (twitter, Instagram you can choose)
Dating site (tinder maybe)
Blind date
College roommates
Mother/nanny
Advertise housekeeper or maid
Affair
Random phone calls
Strip club
Bar
Prison
incest (sorry if you hate it)
evil queen/child savior
old/young ( huge age difference)
mayor/student
mother/call girl