I don't really think it's worth redoing time scaling. I play with a very large quantity of girls, so I'd rather not decrease the focus to a small number of girls, but I can see why someone would want to.
One thing I'd liked to have seen was a level requirement for entering the next brothel. I use a system for this, actually - I don't graduate a girl to the 2nd brothel until she reaches level 7, 3rd = lvl 13, 4th = lvl 19, 5 = lvl 25, and finally you can't enter the last brothel until level 30. Probably my biggest annoyance with my system is that I can't see what level everyone is on the main management screen, so I can't just arrange everyone by their level.
What would probably work best for making the game stay interesting after the beginning is to have a slow progression into whatever it is you want a girl to be. Kind of like what you see in Slavemaker. You would keep a girl on a certain job to obtain whatever traits and have them turn into something, like a bimbo, whore, MILF, maid, sword girl, cat girl, demon girl, stuff like that. So you could have a girl start out as a Tsundere/Yandere and decide you want to turn her into a maid, but instead of just throwing on items like you do now, you would have to make her love cleaning and service by repeating tasks or types of sex, and maybe have certain equipment (maid uniform).