I've been kicking this idea around for a while now and figured I'd see if there was much interest.
A group of four (or so) gets caught in another universe where the typical Hentai shenanigans are common place. The group finds themselves physically changed into something more world appropriate (males can become females, or futanari and vice versa), and the sense of pleasure heightened. Their only way home is a Goddess who has dominion over the entire universe and routinely plays with the people who become trapped in her realm. Will they get home, or will some of the members find that they would just prefer to stay in a world where fantastic sex is as common as water.
I kind of plan on having a multiple world approach to this instead of just a single world, each world would be a familiar Hentai world (Angel Blade-esc, fantasy, Anti-Demon Ninjas...things like that) and when the Goddess is bored with the group in one world she sends them to another. For the most part the Goddess will be the bad guy but not really a beatable one...if you are familiar with the Slayers universe it's like using Dragon Slave to beat Shabranigdo (a spell that falls under his domain being used to beat him...it just doesn't work), but she can be appeased and perhaps if they can convince her to send them home.
With the multiple worlds in addition to being different in theme, I figure the characters features could change if the players wanted it. It's not something I want to dictate, if you get tired with a form and want to try something else for your character be my guest.
Also I'm kind of thinking that this would be a four person game (excluding myself, I am going to try and run it) and while I don't think I can adequately run more than one group if there is interest in people playing the denizens of the Hentai-verse I would welcome the help (the Goddess could have 'generals' that she keeps resurrecting as they expire).
Let me know if you are interested and I hopefully will be able to make this enjoyable for all.