While there's nothing wrong with the current graduation system, I'm kind of curious: I assume idols in real life sign some kind of contract about how long they have to stay? I know K-pop idols sign a 7 year contract but I don't know about Japanese idol groups. I don't imagine an idol can stick around for only 6 months then be like "I'm outta here y'all bye!" and suffer no consequences for it. After all, a lot of money is invested into training a new idol. Any company would want that idol to return them a profit or pay a fee if they leave before their contract is up.
For the final game, I think it'd be interesting to have a little more gameplay surrounding graduation. For example, being able to haggle with idols over their graduation, or know exactly why they are wanting to graduate. Kind of going off of 8thPrince's ideas above. Such as, "I feel my mental health isn't being taken care of in this group" or "I feel I never get center position," maybe you can fulfill her wish to keep her from leaving. Or maybe an idol says, "I'm too old, I think it's time for me to go," you could offer her her own TV show to get her to stay longer, or maybe have her work at a very slow rate so she can have more of an outside life in her late 20's.
It seems to me right now that idols leave after some unspecified period of time and I don't know why or get any input in it. Seems as if it'd be more complex in real life.
Also I'd love to see more consequences and drama surrounding the election. For real idols, it'd suck to go down several spots, or to not even be in the top 10. It might make them rethink staying in the group. Maybe other idols who place in the top 3 will expect more money or opportunities. In the game right now, after the election everyone goes on with life as usual, except some idols have a few stars of fame added.
Anyways I'm super excited to see how fleshed out the game will be with new writing added! The core gameplay is really fun now but it just lacks that replayability without the drama, struggles, and pretty much humanity of the idols and their fans. Not sure if this is an unpopular opinion but I do hope a little bit of darkness is added. I know many real idols go through terrible things behind the screen of cute costumes, pop-y music, and smiles. I'd love to see some light shined onto that.