My picks would be:
1. More locations - e.g. hotel room, office, mansion/penthouse bedroom, gym, locker room, classroom/headmaster's office, strip club, indoor/outdoor swimming pool, picnic by a river, luxury yacht, alpine cottage with a roaring fire and snow on the windows...
2. More positions/variations on positions. Would be great if guys could use some of the girl-girl stuff too.
3. Solo animations - e.g. dances, static poses, masturbation, playing with toys, etc. Would be great if the girl could give her partner a lapdance too
4. More 'voices'. I don't mean dialogue or that I want to hear the girls say 'fuck me big daddy', but more variety of moans/grunts/groans would be cool. Some people are quieter, some make more noise, some have higher pitched moans the closer them come to climax, etc. Would be nice to hear this being reflected in the sim and would make the girls feel much more 'different' from each other if you could choose different sound sets for them. The existing sounds are a big part of what makes this sim appealing to me by the way