I don't so fandoms. I generally write atleast a brief paragraph but sometime I do onliners. RP's don't have to be sexual in nature but I don't mind if they are. Some pairings I've thought of include:

Father/daughter
Brother/sister
Kidnapper/hostage
Boss/secretary
Teacher/student
Master/Maid 
Prince/maid
Casino Owner/Cocktail Waitress
Car Mechanic/Rich Woman 
Billionaire Son/Nanny 
Rebel/Princess 
Personal Trainer/Client 
Crime Boss/Lawyer 
Crime Boss/Police
Mistress/Bodyguard 
Mistress/Butler 
Nun/Priest 
Nun/Demon
Step-Father/Step-Daughter 
Knight/Nun
Police Officer/Prostitute 
Client/Stripper 
Club Owner/Dance 
Principal/Student
Married Man/Married Woman 
FBI Agent/Protect Witness 
Villain/Hero
Son/Mother 
Aunt/Nephew 
Cousin/Cousin 
Scientist/Monster 
Villain/News Reporter 
Any pair where on person might have power over the other