Sounds like a pretty interesting concept for sure, and the images you posted look good as well. Some thoughts I have that may help:
As far as the girls, I would say to let the user customize/create a girl and have the stories follow that character, or if you want to have multiple girls allow certain customizing options such as chest/ass size/shape, hair/eye color, hair style, etc... The more simple stuff. The more in depth options like cheeks, mouth, etc... would be nice, but not strictly necessary.
as far as fetishes... I always love a good upskirt, particularly the view while bent over and masturbation with a skirt.
Games tend to go too far with certain fetishes too, such as BDSM. They tend to either have none, or go all the way to being tied to a table while whips and chains are introduced all while being talked down to by a dominatrix. Personally, being tied up, or tying someone up while just teasing and maybe a small bit of pain is the way to go. sensory deprivation is another thing that is rarely explored.
I'd give bonus points to any game that allows you to scale your own BDSM experience by actually interacting with the environment. Want to go all the way? Go to the closet, grab the chains and attach them to the walls and the girl yourself. want just a little? Grab the ropes and lay her on the bed. tie her up and touch her in the right spots (even more bonus points if you make it so the "tease gauge" goes up the closer you get to the sensitive spots, but touching the sensitive areas raises the orgasm bar, or whatever you wanna call those things.)
details.. details take more time and are harder to design and code. But that is where people form their attachments to the games they play. That is what creates the hardcore fans of a game.
Just a couple thoughts from a gamer/coder who has been around and seen some things.
