The box theory is a bit more complicated than empty space but there being 108 stages wrecks its asshole with a little bit of deductive reasoning. They did more than just withhold those five extra stages, Sakurai pretended 103 was the final count. Those last five stages wouldn't have been hidden if they didn't spoil something. The likelihood of one character and an echo with a stage that already perfectly represents his series covering those five stages is unfeasible.
This supports the idea that the fighter of the day updates leave room for seven characters we don't know about, not two. It also further supports Vergeben, who has claimed Ken, a new Square character, a Pokemon, and something Minecraft related are in the game. Many have assumed the Minecraft representation is a stage, if Nintendo worked with Microsoft, it's pretty obvious what that means. It's unlikely the new Square character is FF, because Cloud specifically reps FF7. Sakurai said he's using the ballot, and there's really only one Square character who was topping polls. He'd likely come with his own stage.