its not just completing the worlds!
this has been confirmed by The Wertle! he has the KH2FM+ ultimania
to get the new secret ending on critical mode!
you need to have all three crowns by that i mean you need to obtain the gold crown!
bronze crown-silver crown-gold crown
to get the gold crown simply defeat the ES,OBTAIN the PROOF OF VICTORY keyblade which you get when you complete the MUSHROOM XIII side quest and defeat all the ORGXIII DATA!
the news about gaining it only by completing the worlds wasnt confirmed and thus not sure! but this was the real info to obtain the new secret ending!
if you have already done this i think you have to wait for the secret ending to reach the point where the TITLE BIRTH BY SLEEP appears!