Only characters that seems likely for are PTM, Hugh, Shredder, and Vlad.
Playtesters think the latter two feel like playable characters (though we don't know if they're like the diet fighter ATs who just have 6 moves at best), while the fomer two have several leaked renders in the style of playable characters.
Or it would have 25 characters because the first Smash just being a weekend project would be impossible in today's game development climate, and the secondary Pokemon rep is more likely to be Eevee, Charizard or Lucario.