As predictable as it is, porn with a short story line is a real turn-on for me. Particularly those in which there's a lot of filthy talking between the actors and they're staying in character, for example between a teacher and a student.
While these aren't unique, role plays that I really want to act out and fuck to:
Locker room sex: athlete + cheerleader
Clinic sex: doctor/nurse + patient
School sex: teacher + student
Office sex: boss + secretary
Plane sex: flight attendant + passenger
Home sex: MILF + son's friend
Beach sex: lifeguard + beachgoer
What's your fantasy?