Here's a story I've used before, but have recently been wanting to use again. It's quite heavy on sex, though that's not to say it will consist of just one long series of sex acts. I would be happy to discuss ways in which to inject more story.
It's a story as old as, err, angels and demons. The two are at war, the infernal and the divine. It's a war wherein dying is usually preferable to capture. For demons, the punishment as deprivation of their sins and vices. Those who prefer to murder and rape are kept caged, while those who swear by gluttony and avarice are deprived of their excesses. For angels, the punishments are reversed. Angels are subjected to and forced to witness and perform the most depraved, sinful acts.
While there are many variations of this basic setup that could work, I want, above all, something involving a demon of pure excess making an angelic prisoner her very own pet. She would be a Queen of demons, with a fondness of sexual deviancy, of bondage and other tortures, of avarice and gluttony.
There is essentially no limit to how extreme this story could get. I wouldn't want to alienate anyone because they think they're signing up for a story of pure gore, however, so I will say that extremes aren't a requirement, per se.
EDIT: Due to overwhelming interest, I can no longer start games based on this idea. HOWEVER, if you have an idea similar to this, that you think I might appreciate, that is still different enough to qualify as a different idea entirely, feel absolutely free to share!