/isn't sure if there's anti-necro rules in place so apologies in advance if there were...
A few things I'd probably want to see in future installments, myself (besides the L4U ability to change lyric order), would be...
- Ability to change hairstyles if the character has alternate styles... which many characters seem to now, esp. when you include Vivid Bikini's changes for the DLC. Granted, not every character can take advantage of this too well... but I think the only ones who probably would get NO POSSIBLE ALTERNATE VARIATIONS WHATSOEVER would be Haruka/Yuhiho and maybe Hibiki (unless you count barette removals for Haruka). I'll admit for some characters (hi Mami, hi Makoto - basically characters with grown-out hairstyles) this may run into some issues though.
- Colorful Days to become the 12-color version, to accommodate Takane and Hibiki's additions into the main cast (I know full well this one is not happening because it would be cost-ineffective in general - they'd need to rerecord for 9 idols AND have to make animation changes, and what if more idols get added to the main 765 roster?! - it's just me being more than a tad nitpicky)
- English subtitling options - NOT A FULL-BLOWN ENGLISH-LANGUAGE RELEASE, MIND! (I personally would not want one, especially not if we're to have a repeat of Shiny Festa as a result. I don't own iProducts to begin with, so the release was completely useless to me...) Just, I have noticed that there are some Japanese games providing subtitle options in other languages as of late. (Granted, they're typically from series that already have large overseas franchises to begin with, but...) I think this would be a suitable compromise - even if the quality isn't too great, so long as it can be understood. Besides those hoobies, I didn't notice any GLARING issues from the translations of Shiny Festa that I have seen, and I (personally) don't mind not having the song subtitles translated too much.
EDIT: Also for Takane to have an ACTUAL IMAGE SONG of hers to be playable in the main series, because as far as I can recall she is the only one who does not. (I'd say Hibiki too, but because Brand New Day... and yes, I know she gets Kazahana in the Shiny spinoffs, but that's why I said "main series") I personally can see Flower Girl being a really cute song to have for a majority of the idols. (Especially Makoto. >>; I'm sorry, I can REALLY see her being a fan of this one)
...though if the scoring systems stay as dull for slow songs as they were in iM@S2, that would be of no use x.x