The thing is, unless you cant play Smash Down until you unlock all the characters, its not gonna matter anyways. If you start wirh the OG 8, unlock 1 character, then play Smash Down, then the entire theory falls apart because 9 is nit divisible by 4. Youre pribabku not gonna unlock 4 fighters at a time either. The game probably just ignores the remaining 1-3 characters and calls it a day for Samsh Down.
I dont buy box theory, because the special editiin revealed at the 8/8 direct didnt have all the figjters on the cover since its a placeholder. No one worried then. This new box is also more likely than nit a placeholder, so why worry now?
Coro coro usually recieves early information from Nintendo (didnt they reveal sun and moon pokemon all the time during pre release?) So I think Coro coro is the most reasonable "theory" of the ines we have. Besides, i just find it odd that there is THAT much negative space in the select screen. 3DS had a blank button for dlc when it launched, but Ultimate wouldnt need a dlc button the size of 6 stage spaces.