Teacher/student idea, full concept is under the cut.
For simplicity's sake, I'll be referring to the student as male, and the teacher as female. He's a young (16-19), somewhat sheltered/distant, immensely wealth teenager. His parents believe, for whatever reason, that it's best for their son that he's tutored in all subjects by only one person. They hire her, an older, brilliant woman who is now responsible for this boy's entire education. As his parents aren't around very much, it's also up to her to help him grow up. As time goes on, they become more and more attracted to one another, and their relationship turns sexual, even while she's giving him a top-notch education.
I'd be interested in really playing with their relationship (based off of how Alexander the Great was supposedly only tutored by Aristotle, just through the lens of RatedX). He doesn't have a whole lot in the way of peers or companions, and his parents view him more as an asset rather than a person--so he's really latched onto his tutor. Mommykink could also be a lot of fun. She could be a recent hire (maybe they've gone through multiple tutors that don't meet their standards), or could have been working for the family since he was younger.
My preference for genders is how it's written--male student, female teacher, I'd like to play the student. I'd be willing to discuss playing the male teacher against a female student, or making it femme or slash.