I have nothing but praise for this game. I've played it a few times and it's ended up being the only store management game to hook me. While it is a little slow and really easy once you understand how the game works, it is definitely worth the time of anybody interested. Here are some ideas I have had while playing:
Cosmetics:
I don't have a ton to say here, since many others have already touched on this topic. The current customization options are great, but adding more would only help. This could be expanding upon existing options, adding new cosmetics like hair/body color, accessories like horns or glasses, size of tits or ass, or even giving other options for girls entirely! Others have talked about new girls having other personalities, but I'll get into that later.
Difficulty:
It's difficult to make a tycoon game have "difficulty" without being annoying, as once you get off of the ground it is hard to make the player lose momentum in a fair way. But some other people have had good ideas, including timers on customer drinks, a buff to the amount of happiness lost while she as full, or losing money when scraping drinks. This could also be tied into those other personalities of girls, with some being shy and receiving less happiness from being milked, or others being really assertive and upset when they aren't close to or at maximum size. But while these would all help a little bit, the main problem is trying to stop the endgame from being too easy. This could be solved in a few ways. First, you could make it so that the longer the girl is full, the faster she loses milk. This could make it so that you couldn't leave her to create a complicated drink while she is full, and need to address the problem immediately. Second, you could make it so that she slowly leaks breast milk into the drink while full, which could interrupt a customer who doesn't want breast milk. My last idea is that you could make events like the bursts of milk that swell in take large chunks of happiness if they happen while she is full. These would all help because even in the hyper version of hard mode, when her milk gain overtakes her maximum size tenfold and she hits maximum size the second you stop milking her, you can kind of just ignore her pleads to make a drink, then refill her happiness by milking her for two seconds. Those changes would make that mode more of the intended race-against-the-clock feel instead of constant free upgrades.
Those are my biggest additions I have come up with, I just hope that you are reading everybody's posts and continue to update this wonderful experience. I had a blast with this and cant wait to sell more drinks.