Sorry for the late reply - I was sick and busy this weekend... a really miserable combination.
I had never seen Jack-O-Nine before- interesting.
You have some great ideas - I plan to introduce random events as a major part of the game to fill part of the role of quests you mention. I have actually been thinking that the missions might have been a mistake to implement. I want to game to be distinct from the whole Fantasy-RPG genre, with a heavier focus on economic decisions, but I do want the economic focus to be balanced by a small group of girls that you keep and develop as a permanent set -- the tradeoff being that as you develop your close girls, they also become more valuable to sell off.
Another thing I would really like to implement later to improve on the immersion you mention is to give the girls personalities, but I want to do a good job with that rather than just have a few stock lines they repeat (as happens in some games). I'm not sure how that will develop... lots to do before I get around to that.