Please look at the side of the box. People have been discussing it on twitter, but missing the significance of the ordering. Here, forgive my color-coding:
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I took the liberty of adding Isabelle. Do you see the pattern? Here, let me help:
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The fighters are in groups of 9, presented exactly the same as on the website and all other promotional materials. (Echoes in order, Miis and PT as single entries) The promotional images used on this box were clearly taken from a source location in which they were in sets/rows of 9.
This means we know what the other 4 sets of 9 looks like:
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It's all very clean and works perfectly. With two big implications.
There is no room for an echo in any of the first 3 sets of 9. No Dixie, no Shadow, no whatever. It would throw everything off, and make the side of the box we can see look different.
But there is exactly one unaccounted for space in the final set of 9. Somewhere, there is an echo between Palutena and Cloud.
This is the other half of the box:
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And of course, there is only room for one blank space after Isabelle, so that leaves one new character--possibly the Gen 7 Pokemon Vergeben alluded to.
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It's always possible that this is just a box, and the product designers chose to only show off 72 character slots arbitrarily.
But realistically, it looks like this is our roster.