follow-up: it would be super cool if, in order to get the whole story, you had to play through each datable character just like in some dating sims. like the main plot is obviously going to be about yamane and senpai, but maybe there are larger puzzle pieces going into play. Maybe there's a big scandal going on at the school and if you have all the pieces to the subplot you could frame/expose someone big ? or maybe after achieving each senpai's true ending, something about the school changes slightly, almost unnoticeably. once you have all the good senpai endings you play one last time and uncover some huge mystery about the school because all those small changes were actually evidence to support rumors of a big crime that happened. completing this route would give you the true ending to the whole game.
alternatively the extra unlockables could be used to romance a mystery character! perhaps a sickly girl who was released from the hospital or a transfer/exchange student?
personally i like the first one but either way it would make for an awesome end-game.