Well I am the type of guy that likes to hear ideas and then put both ideas into one, but as for ideas I have a few they are as follows.
Two siblings separated at birth and have no idea who each other exist and then they get together and hook up. They have sex and after wards they find out they are related. She doesn't want to continue, but he uses his skills to get her into a complete slut
The second on is that your character is a good two shoe, but she meets my guy and then her submissive sides resurface as she spends time with him and then they begin a slave master relation ship