While it is possible there are remaining characters, I doubt Ridley could sneak behind ZSS : it would break the roster's pattern
Now you could say "But the roster's shape could completely change !"
However :
We hit a wall if we don't add another column ; Bowser Jr, Wario and G&W push DK and Diddy to the next row, Ganondorf helps pushing Pit and Palutena below.
The CSS follows a logic as many found out in the Leak thread ; the pattern won't break, EXCEPT if there is another column added. If that's the case, it'll either be K. Rool or Ridley getting the spot, and Mario and co. getting another rep to fill the top row.
It's not impossible, but Ridley is in a tough spot actually...
We'll see in ten days anyway.