Best 51 character roster imo.
Reasoning on non-Gematsu characters:
Dixie Kong: I agree with many on having a third DK character. The DKCR series has brought up the series' relevance. Dixie Kong seems like the best bet because it adds another female to the roster and that she is more relevant than K. Rool. However, he could still get in over Dixie.
Ridley: The teasing. The Pyrosphere was confirmed almost 10 months ago and we STILL don't have any solid evidence of his possible inclusion as a Boss. Due to these multiple teases (especially the shadow in the Direct), I feel that Ridley still has a shot as a playable character.
Dark Pit: First of all, in the 3DS arrangement here (
http://smashboards.com/threads/51-characters-total-reasoning-inside.358059/page-9#post-16921670), a third KI rep fits the best right next to Palutena. Also, in every smash game, there has been a series that has gotten two or more newcomers (SSB64: Mario and Pokemon, Melee: Mario (3 newcomers), Zelda (3 newcomers [4 counting Sheik]), Pokemon (3 newcomers), and Fire Emblem, and Brawl: Kirby.), and this hasn't happened yet in Smash 4. We already have Palutena, so a second KI rep will make the pattern go on. Dark Pit seems like the best bet here as a new villain and the fact that he was teased in the Palutena trailer.
Mewtwo: Mewtwo's relevance has increased with the release of Pokemon X and Y. He was one of two pokemon (The other Charizard, a playable character) to get two mega evolutions. With the added fact that Mewtwo has not been confirmed a Pokeball summon or a trophy, Mewtwo chances are pretty high this time around, unlike Brawl.