As others have said between Namco Bandai and no SSE I can't imagine anyone being cut for time unless Sakurai deems them super low priority for w/e reason. I do wonder if Squirtle/Ivysaur could potentially be replaced ala Young Link -> Toon Link with other fully evolved versions of starter pokemon even if its just Blastoise and Venusaur. Could be a way to make all 6 gens playable, this might be problematic if they did decide to bring back Mewtwo
Gen 1: Charizard, Pikachu, Jiggly, maybe Mewtwo
Gen 2: Meganium/Feraligator
Gen 3: Blaziken (if any type was to get two pokemon it'd be fire with Blaziken), Sceptile, Swampert
Gen 4: Lucario
Gen 5: ???? (Can't really see Surperior or Samurott being playable, no idea besides Zoroark who could be playable)
Gen 6: Geninja
I dunno just out loud thinking Pokemon will probably just be Pikachu, Jigglypuff, Lucario, Charizard, Greninja, maybe Squirtle/Ivysaur or a replacement, and either returning Mewtwo of surprise newcomer Blaziken
While 7-8 may seem like a lot Brawl did have 6 out of 39 be pokemon I can see if the roster is 45+ pokemon getting 7-8
Everything is Kirby Fighters!
Ice Climbers got cut like Knuckles and Ike, will be very upset when Buff Climbers don't show up