http://i.imgur.com/abw0VWV.png
So this is the leaked CSS (character selection screen) we've been working with. I've seen (and believed, for a while) some theories that it fits with an earlier text leak claiming all the same characters, plus Ridley, Mewtwo, Dixie Kong, and the Chorus Kids, as they could take up the empty spots on the roster.
However, when we try to actually fit the characters onto the CSS, we run into some problems. In particular, notice how Row #2, containing the DK, Zelda, and Metroid characters, is full. There's no room for another rep for any of them, such as Dixie Kong or Ridley.
By all appearances, the leaked CSS works like Brawl. The characters are in a specific order, and the first row displays the first set of unlocked characters in order, then the second row displays the next set, and so on. As a result, when you unlock a new character, it shows up in its assigned spot, usually somewhere in the middle, then pushes characters after it further to the right and typically pushes the last one down to the next row. The Smash 4 demos seem to work the same way, as the LoZ characters are split between rows as if another character is supposed to push the first one down to the next row to join the others.
So let's say this CSS works the same way. Adding any characters to Row #2 would start pushing Metroid reps off that row and down to Row #3. So the Metroid reps would be split at the scroll line - inappropriate for a final Smash roster. The only way to work it out would be if there were at least two characters added before the Metroid reps - say, Dixie Kong and the Chorus Kids. This would push Samus down to Row #3.
Donkey Kong | Diddy Kong | Dixie Kong | Link | Zelda | Sheik | Ganondorf | Toon Link | Chorus Kids
Samus | ZSS | Pit | Palutena | Marth | Ike | Robin | Kirby | Dedede
This causes more problems, as adding two characters to Row #3 pushes the Kirby characters to the right and gets one of them cut off. Of course, adding two more characters to Row #3 could fix this by pushing all the Kirby characters to Row #4. Ridley could be one, and then another could be a mystery fifth character?
Samus | ZSS | Ridley | Pit | Palutena | Marth | Ike | Robin | ???
Kirby | Dedede | Meta Knight | Little Mac | Fox | Falco | Pikachu | Charizard | Lucario
It just keeps happening. Now the Pokemon characters are shoved far right enough to get cut off, and unless we end up with a good bit more than the apparent 54 characters, there aren't enough remaining characters left to push them all the way down to Row #5.
There's no getting around it. If the leaked CSS is accurate, then getting even just one more DK, Zelda, or Metroid rep would mean that even more sets of characters are getting split apart one way or another. Whoever the others are, it'll still happen the same way. At least, unless the characters break completely from the Brawl scrolling pattern and several of them do way more jumping around than ever before. Neither of those are particularly plausible. If Sakurai was planning on including those characters, he'd have a CSS arranged very differently to account for them.
For the moment, I'm staying out of the dispute of whether or not the leak or roster is accurate. But, with all this in mind, I feel confident in saying the leaked CSS is not compatible with any DK, Zelda, or Metroid newcomers to fill the remaining space. Including the ones predicted in the Ridley/Mewtwo/Dixie/Chorus leak.
Funnily enough, the original claim about Lucas, Wolf, Ice Climbers, Snake, and the Chorus Kids taking the remaining five spots? They could actually fit into this CSS just fine.