Okay so, little theory about the Rhythm Heaven character removal in Smash 4:
Everybody knows Ice Climbers weren't brought back for Smash 4 because of the technical limitations of the 3DS, and
assuming the Gemetsu leak was true of there initially being Chorus Kids planned as a character (thus
what the emblem data in Smash 4 was representing), maybe the reason for that RH character getting cut was because it had technical problems of having even more characters on screen at once than the Ice Climbers.
However, looking at the emblem data that references Rhythm Heaven in the first place, there's no character reference to the Ice Climbers (as well as Snake from Brawl). This could imply that at this point in development, they had already scrapped them for technical reasons, but could have still been working on the Chorus Kids character. Thus they could have been scrapped for another reason, be it time, moveset implementation difficulty, etc. After all, they weren't creating other new "multi" characters Rosalina & Luma and Duck Hunt Duo in the style of the Ice Climbers, so why would the Chorus Kids be any different?
A few other things: looking at leftover Smash Run enemies for both multi-characters, maybe those were still included because they had been started earlier in development and/or were too easy to continue development on anyway. And maybe the Chorus Kids were meant to be visually flat like Game & Watch, at that matches both the RH trophies and Smash Run enemy in the final game.
What do you all think? Maybe we could have a little solace in terms of what to blame for the RH character not getting in in Smash 4.