Hello. I've been playing for about a week, and here are some things I've thought of.
1.
It would be nice if you could set rules, per brothel, about what is and isn't allowed to happen in there. Like a list of every type of sex act in the game, like BDSM or lesbian sex or bestiality, and turn them on or off individually. There'd be a price to it, as whenever a customer would want to do something that's forbidden, he (or she) would be turned away and you wouldn't get his money, but on the other hand maybe the girls who don't like that stuff would like working there better and get a little happiness boost. But even if there's no upside to it, I'd just like to be able to set rules so maybe I don't have to force my girls to have sex with animals or whatever.
It'd be cool to be able to set this per brothel, so maybe you could have like a regular house with everything turned on, and a house that has only lesbian sex turned on, and you send all your lesbian girls there, and so on and so forth. Maybe it could end up with the all-in-one, anything-goes places making more money, but the specialized places having higher customer satisfaction and happier employees. Also, it'd make buying several brothels more interesting than just getting more room for more girls.
In addition, it could be a way to further differentiate girls working the brothels and girls working the streets, whereas customers in brothels have to abide by the house rules, but customers in the street don't have to obey anything and can try to get away with stuff.
2.
When a girl dies, all her stuff and money vanishes, which is weird. When a girl dies, maybe you could be given the option of taking her stuff, but that would be an evil act. Except if the girl's a slave then you inherit her inventory by default at no cost to your morality.
3.
It'd be interesting if girls could "buy themselves". Like, if a free girl has enough money to buy her own worth (and the appropriate personality, according maybe to the Spirit or Confidence stats), she could literally make you an offer to buy herself back. You'd be given the choice to accept or refuse, or even ask for more money. If you accept, she leaves, you get the cash and maybe the other girls get a bit happier, and it counts as a good act. If you refuse or ask for more, she stays, keeps her money, loses happiness, and it might count as an evil act.
Of course, if a slave girl somehow has enough money to buy her freedom, you wouldn't get a morality penalty for forcing her to stay (since that'd probably be money she stole from you to begin with). But it'd count as a good act if you let her do it anyway.
On a related note, maybe sometimes a customer could have a good enough time with a girl to want to buy her from you. He could make you an offer, or you could name your price.
4.
Every once in a while you could be approached by a girl who actually wants to work for you. It could be affected by your fame and moral alignment (i.e. famous and nice players would see that more often than unknown and cruel players). It'd be a nice change from how things are now, where pretty much all girls are forced into the job.
5.
I understand there's a lot of work to be done on items, but it would be nice if there was a limit to certain types of consumables. As things are now, there's effectively no difference between a +20 item and two +10 items, and if a girl's completely exhausted, you can just pump her full of cheap relaxers and she's back on her feet instantly with no side effects. There could be a "Medicine" item category, where you can only give a girl one of that type of item every turn. That'd make the expensive, more effective items more valuable. And it'd be a strategic decision too. If a girl is tired and wounded, you would have to decide if you give her some healing or some relaxant, instead of just throwing all the pills you have at her all at once.
And it'd be just that specific type of item that would be limited, so you could still give her all the clothes and jewelry you want.
6.
Kind of related to that, there could be a separate "Food" category, where if you give her too much of it, there's a chance she could acquire a "Plump" trait. Something like "This girl has some meat on her bones, but she's still pretty. (fetish)". And if she already has that trait, it'd get replaced by "Obese", "This girl is a whale. (-looks) (fetish)". As things are now you can just feed your girls tons and tons of candy and they just get happier.

7.
Speaking of traits, I'd like to see something like "Exotic: This girl has exotic features. It looks like she comes from a far-away land. (fetish)". It kind of leaves it open to the player's imagination, whether the girl has has rounder or almond-shaped eyes, different skin color, or any sort of "ethnic" physical differences one would want to think of.
Alright then, that's my two cents for now. I'm looking forward to see where this game goes, even if none of my suggestions make it. Good work so far!