In one corner, actual promotional material for the game.
In the other, chairs in a presentation's backdrop.
Which do you honestly think would be more likely to hint at unrevealed content in the game? Be real here.
For my take, I think this one is worth looking at. A post I made on another site:
Now, notice that it starts with Luigi at the top and then goes in sequential order. It's missing 9 characters right away - the original 8 plus Dark Samus. It then continues for another 9 characters, up to Sheik, before breaking. 9 more characters are missing, from Zelda to Mewtwo. Then we get Roy to Snake, 9 characters. 9 more are missing, Ike to R.O.B. Then we get Toon Link to Mii Fighters, another 9. Then eight characters are missing, from Palutena to Cloud, suggesting an echo fighter of one of those 8 (Palutena, Pac-Man, Robin, Shulk, Bowser Jr. Ryu, Cloud). Then we have 9 more slots, with Isabelle missing and one more unknown.
If this is supposed to include everyone and just has unrevealed fighters edited off, this would mean we're getting one more newcomer and an echo fighter of one of the above 8.
This looks like a very deliberate design to me, so while it's technically true that it can still be edited, I don't think it will be. Would they really resize it just to make it look like this is how it will look when it actually doesn't? Everyone is saying it means nothing, but I think people just don't want to believe it because it's disappointing. It is also possible that just not every fighter will appear on the box, but then why design it like this?
What I forgot to mention here is that those missing characters are almost certainly on the other side of the box that we can't see.