I'll try to explain the box art theory as simple as possible in this thread for those who at least want to know how it's laid out.
This is the official box art released.
This is the full roster using the simple pattern found.
It's really quite simple. They divided the characters into groups of 9 to divide evenly into the 72 slots on the box art(36 on each side).
The groups of 9 are alternatively printed between the left and right sides in their chronological appearance to the series,
with the exception of echo fighters which are placed next to their respective fighter.
The box art follows this exact pattern to the Tee, EXCEPT for when it reaches Corrin. Using the pattern, Corrin should be on the left side of the box with Bayonetta coming after the mii fighters but clearly he is not.
The only way this could happen is if there is an unrevealed echo fighter in the group of 9 fighters where Corrin should be, forcing him to the right side of the box into a different group, since we know echo fighters take precedence over the chronological order in which a fighter appears for this box art. The pattern continues perfectly, otherwise.
This is the main reason why some people think the final roster on the box art is real. It doesn't make sense for a place holder to bother moving Corrin for an unrevealed echo fighter. We'll have to wait for the next direct, though.