Oh, something else that I thought might be cool is the concept of "preferred positions". You can currently choose where they stand on stage and I think it's quite detailed already but adding something like stat boosts if you put them at their preferred position would be cool (e.g their preferred position is to the left of the center)
Following the concept of "preferred positions", there could also be like "roles", e.g main vocal, main dancer. If they are the "main vocal", their "vocals" stat has more weight on the success of the single/performance and for "main dancer", dance stat has more weighting. The 'leader" role could be done based on age/experience, the more suited they are to it, the better overall performance the group would give. The roles should be optional but if you do choose to use them, the idols who are the "main vocal" or "main dancer" should get more exposure and the others should get less.
I've been playing a lot the last few days and have been enjoying the game quite a bit. Looking forward to new updates! Even more excited for the switch version :P, keep up the great work!