Please tell me this was revealed at EVO & there was someone recording a live-reaction of the audience.
Also, why can't I find more on that song used in the video?
This tweet from this afternoon makes so much sense now.
Oh yup, because this is also to celebrate 10 years of Idolm@ster. Maybe for God Eater's 5th or 10th anniverary, they'll do a costume collab, maybe with Soul Calibur (which might make more sense given that series uses weapons like in GE) or again with Tekken.
............also holy crap, Idolm@ster is 10 years old.