Well, this one is going to be a bit unusual for me. I've been toying with some ideas that are all a bit unfamiliar to me, things I haven't really done in the past, and although there are many, many variations that could be used, I'll only describe one of them in any depth here.
Basically, you have a mother, and a son who never knew his father. Which is probably because his mother is, in fact, a rapacious fiend of sex and lust - a succubus! The son has no idea of that, however. The only hint he has ever been given about her true nature, is an unusually large number of vivid, wet dreams involving her, usually being utterly dominated by him. Obviously, exactly the opposite is true - while she hasn't touched him physically, the dreams are her doing entirely. In other words, he has no clue, not until his x birthday, or some special event occurs, and she reveals everything to him. Unlike in his dreams, however, it won't be he that's dominating the mother, but quite the other way around. I could play either role, and while it isn't a requirement per se, I would strongly prefer if the succubus mother were also a futa. Just for the sake of clarity. And hotness.
Other variations on this theme would be to do the same thing without the element of incest, or without the succubus bit. Other ideas you might have gotten that resemble it but are different in some way, are also welcome.