I like the idea of BoneBrothel. You could actually make it an MMO too. Check out Newgrounds for "Sim Brothel" for an idea of where to start... then kick it up a notch and make it much more expansive! I'd like to see multiple citys/destinations... customizable buildings/rooms! Furniture selection, room upgrades.... a wide variety of randomly generated girls with different stats and skills! If not actually an MMO... the ability to visit a friend's brothel or trade girls! All that said, I'd probably be too uncomfortable to ever actually interact with people I know IRL through said game.... but that doesn't mean there aren't less shameless people out there! I'd enjoy a more substantial brothel sim anyway.

Also, why not make a game about the world of competative and erotic Female Wrestling. Like Rumble Roses XXX, but hardcore! Customise your own female wrestler and claw your way to the top of the leaderboards! BoneSlam!