You could add "After story" map, where you can find all the girls whose story you have finished.
So instead/addition of FAST SEX option, you can visit them and maybe train them.

You could also add system that they react differently, depending how much you have visited them and what you have done to them.

You could also add statistics for each girl so you can see, what kind and how much sex you have had with each one.

That's an interesting idea kinda reminds me of a Hentai Game I played some time ago.

As I was exploring the the game further it occured to me that the current story lines can be expanded. If an interface can be created to share stories and poses that go along with it. Or as a first step customizing the models face, hair,breast,waist,hips and legs. These characters can than be exported to be shared or imported into the game story to create a new experience for the player. On the subject of hair ,adding pubic hair of several shapes would be a nice addition imo. Ofcourse some of this have been done by the Japanese gaming companies notable Artificial Girl 3 by illusion. However the physics in Xstoryplayer is definitely a cut above everything out there at the moment.

Perhaps I am being greedy but the scope of this games future expansion has me quite excited I must say