I remember reading a post like that aswell. i think that was already some time back though...Last I heard, the plan is that someone new will join, once one of the other main girls' stories is complete. Managing 8 main girls, along with all of the other side and background people, is about where he feels is his limit is right now.
still, there are some girls i wish were promoted to main. i especially like kate and juliet. might be interesting to get them both in one room similar to Emma and Felicity...
Vanessa or Hana are also interesting characters i wish would get a promotion... though i would be happy just getting more options for Hana, maybe just beeing able to invite her to the dungeon and "teach her more about interesting kinks for her works", also we didn't get to see her stash yet... maybe lucia needs some lecturing in the dungeon aswell? might be funny to get Kali involved there aswell...