Okay so first of all, I'm a beta backer so I could only play it since yesterday, but I'm already kinda in love with this game. It's already really fun to play and shows lots of potential for the future. I would have tons of ideas to make this game even better, but I will tell them one by one.
First of all I wanna start of with the thing that bothered me the most in the first few hours i played this game now. I tried out a lot and also restarted a lot to get a feeling for this game and to get to know it a little better. In every restart I did I always had a problem with the girls' mental stamina, no matter my playstyle. Maybe I'm just bad, but I think so far there's no active way to really handle this is there? What I would suggest for the future are 2 things:
1. When the doctor's office is implemented I would suggest that he can treat both mental and physical stamina problems, so girl's can go to the doctor's office if they need to. (But I don't know what you had planned with it so far.)
2. What I'd really like to see is freetime activities, which would be certain events you could organize, that, depending on the event, better the idol's physical and and/or mental stamina. (For example simple events like a dinner or bigger events like a vacation.)
Also I would cap the mental stamina. At the moment (at least what i saw), it can go into endless negativity and there's no way to get an idol out of this. So at like e.g. -100 I would just cap it maybe.
I also want to reply to likitty1227's idea of the international trait. I think this would actually be a great idea. I would even go a little further. Someone on that Twitter post also mentioned that you could implement an ethnicity system for the girls. I think this would be a great idea! 90% of the girls could still be Japanese, but 10% could be from other Asian countries, like South Korea, China, Taiwan, Thailand etc. In the real idol industries it's often the same. So those idols could have the international trait. To implement this you would probably need a name list for random generation. If you would like to implement something like this you can come back to me if you want to, I would be willing to help you with the name lists.
I will let you know some other ideas over time then.
Anyways, keep up the great work, I'm happy to be part of the beta!