For me, the biggest draw in any game is immersion. Having a story is good, but a sandbox is better because I can then make up my own stories. XStoryPlayer is great because it has the potential to do both - you can have individual stories, but you also have a large city map with date/time capabilities and rudimentary chat AI already, so it seems like 2/3 of the work for a sandbox mode is already done or at least already exists in some form.

So, I would suggest a sandbox mode where the player can pick any character he wants, and then he starts in the apartment of his choice and then has a city to explore with random characters (both male and female) populating it. This would mean you'd need to create more faces, more bodies, more hairstyles, and more clothes, and then somewhat randomly put them together to create a unique NPC. He can go to a local bar, for instance, and try to pick up a woman. Maybe that particular woman will be there one night, but others will be there on different nights. Or, maybe he goes to the local pool on a weekend afternoon and tries there. Or maybe he stays at home and tries to pick up women through the internet. You've already got most of the assets for these last two done. I would give bonus points if it would be possible for the player to create an NPC and place them in their game world to interact with.

As for the actual sex, I think you guys are already moving in the right direction - more realistic movement from the woman, better facial expressions, orgasms - these are all things I definitely want to see. Another thing that I would like is visible hands. If I click and hold on an NPC's breast, for example, I'd like to see a hand/forearm come up and grab that breast where I am clicking, with an appropriate facial reaction from the NPC.