I've always kinda wanted a "Music Video" event to be in the game. I don't know how it would work, though; would every song have a preset video, with the actions of the "story" segments of the video determined by unit placement? Or would the actions be idol-specific? I'll use Little Match Girl as an example of what I mean...
Little Match Girl might start with Leader walking on a snowy street, clutching her hand to her chest as if protecting something. Cut to Second shivering next to a make-shift fire. Cut to Third looking at the lit match she's holding as she leans against a building. Then you cut to the dance segment once the main vocals start.
Haruka will be seen walking down the snowy street from above, but the place at which that cut happens changes depending on her placement. Meanwhile, Yukiho might be kneeling against a wall, trying to sell matches to the passersby. Then you might cut to Hibiki staring in a warm window, followed by a close-up on her breath, then cut to dance. If Haruka is leader, hers will play first of the three, then follow the order.
Alternatively (Character Route B, I guess) there'd be a priority system or something, so that, whenever Yukiho is in the group, her match-selling segment will always be played when they mention the eponymous Little Match Girl. (Not sure that would work given the timing of the backing vocals for that song, but work with me.) Then, they might have so-and-so many others who'd have lesser priority for that specific spot. It'd be complicated, though, so I'm not sure it'd be doable.
Also alternatively (Character C, apparently) the player would be able to pick the order of the "story" segments, and how long they run, a la the lyric choosing in L4U!.
On that note, the choosing of lyrics should return for the next game. Some dances might be weirder, but a lot of them are seriously Leader-biased, and I like my units to be more than Leader-and-backup-dancers. (Also would let you balance out for quintets.)