1. The "Picking who sings what line" thing from L4U
2. The ability to choose the number of idols for your story unit (From Solo-Quintet)
3. More idol interactions in the story; Maybe limited to two idols interacting at a time, with all members of a unit less full-on interacting and more chiming in during Story things. (Just a thought; since I can't play the games, it doesn't bother me as much)
4. Different voice tracks for Mami and Ami; maybe have one as slightly deeper than the other, or different pronunciations, or...?
5. More rivals. Jupiter's cool, as are 876, but the latter is DLC-only, and you only face Jupiter in story missions; otherwise, you face 765 idols only, which I find odd.
6. More "Default" costume types to recolor: maybe a spiritual successor to Punkish Gothic that shares colors across idols? Or... Well, I can't think of much else, but I'm probably just not thinking too hard.
7. More songs that come with the game; the arcade songs should have been in 2, as well as all of the 360 songs. (I mean, why add Go My Way!, relations, and My Best Friend, but not Massugu, Omoide wa Arigatou, and Watashi wa Idol?)
8. Yumeko and Cineria to be ACTUALLY SEEN in 3D MODELS with ACTUAL SONGS. I mean, Cineria's even in the Top 100 (maybe even Top 50...?) in 2!
9(?). A prequel game with Kotori and Mai, as well as one other character. (Not Reiko, as riola is kinda already canon, but they should be a rival.) Maybe one of the older CGirls, like Takahashi Reiko or Kawashima Mizuki. At the very least, I wanna hear Mai's version of ALIVE.