Thank you for taking the time to create such a well thought-out, fully fleshed out expansion game! There really aren't many out there, and everything about this one is great imo; (The art, mechanics, gameplay, etc!) It seems like you may be done with this project, however I wanted to compile some ideas in case you're interested in taking a look, or in case other players also want to theorize.
I was looking through some of the other comments on this game's community page and I think some of these ideas have legs. I really like the idea of adding some form of butt expansion, since it's so heavily associated with breast expansion. I've got a couple ideas for how this could work:
1.) Have her butt increase along with some form of upgrade, like maybe a multiplier for payment, because the customers would find her more attractive and tip more or something
2.) Make it so that as her butt grows, she becomes happier or more tolerant of her imbalanced curves
I also liked the idea of having different girls of different archetypes, such as a gothic girl or more bubbly girl.
Another fun idea is having purchasable alternate viewpoints for the camera, so that even once she's so big that she covers her clothes, you could still appreciate the outfits you buy depending on the camera angle.
Again thank you for making such a fun and high quality game for such a niche community. Have a great rest of your day!