Spyder Enzo, don't be too quick to dismiss Meowth. There is a pattern that Sakurai does base most of his roster choices on the Melee poll. Remember, Sakurai did come up with the roster for Brawl right after he was approached to develop Brawl. He finished his Brawl roster first and then he asked the Japanese fans what they wanted in Brawl.
Red = Melee newcomers
Yellow = Brawl newcomers
Green = possible SSB4 newcomers
MARIO
#1 - Bowser (169)
#2 - Peach (66)
#3 - Wario (65)
#4 - Toad (27)
#5 - Koopa Troopa (9)
#6 - Kamek (8)
#7 - Goomba (7)
#8 - Mallow (5)
#8 - Geno (5)
#10 - Baby Mario (3)
POKEMON
#1 - Mewtwo (35)
#2 - Mew (24)
#3 - Red (18)
#4 - Misc. Pokemon (12)
#5 - Lugia (11)
#6 - Meowth (8)
#7 - Raichu (7)
#8 - Marill (5)
#8 - Bulbasaur (5)
#8 - Togepi (5)
ZELDA
#1 - Ganondorf (36)
#2 - Young Link (8)
#2 - Zelda (8)
#2 - Sheik (8)
#5 - Shadow Link (2)
KIRBY
#1 - King Dedede (46)
#2 - Gooey (17)
#3 - Meta Knight (11)
#4 - Misc. Kirby Companion (10)
#5 - Waddle Dee (3)
MOTHER
#1 - Mr. Saturn (18)
#2 - Poo (6)
#3 - Other Good Guys from Mother 2 (4)
#3 - Paula (4)
#5 - Starman (3)
#5 - Jeff (3)
STAR FOX
#1 - Falco (7)
#2 - Slippy (5)
#3 - Wolf (4)
#4 - Peppy (2)
FIRE EMBLEM
#1 - Marth (21)
#2 - Misc. Characters from Fire Emblem (18)
#3 - Oguma (8)
#4 - Serris (4)
#5 - Sigurd (2)
#5 - Leaf (2)
RARE
#1 - 007 James Bond (35)
#2 - Banjo & Kazooie (18)
#3 - Diddy Kong (15)
#4 - Blast Corps Robot (2)
#4 - 006 (2)
OTHER NES/SNES CHARACTERS
#1 - Sukapon (15)
#2 - Lip (9)
#3 - Prince Richard (8)
#4 - Metroid (6)
#4 - Foreman Spike (6)
#6 - Pit (5)
#6 - Donbe or Hikari (5)
#6 - Misc. Characters from Marvellous (5)
#9 - Samurai Goroh (4)
#9 - Popo and Nana (4)
#9 - Balloon Fighter (4)
NON-NINTENDO
#1 - Chocobo (9)
#2 - Doraemon (7)
#3 - Bomberman (5)
#4 - Crash Bandicoot (4)
#5 - Sonic (3)
#5 - Mega Man (3)
The Melee newcomers he added on his own accord were Dr. Mario, Pichu, Mr. Game & Watch, and Roy. Sakurai added Dr. Mario because he liked his music and added Pichu as a new joke character. Sakurai added Mr. Game & Watch as a wild card and added Roy to advertise the upcoming new Fire Emblem game on the GBA.
The Brawl newcomers he added on his own accord were Zero Suit Samus, Ike, Lucas, Olimar, Lucario, R.O.B., and Toon Link. It seems that Sakurai knew what the fans wanted before he asked them because Ike, Lucas, Olimar, and Toon Link were some of the top choices on the Brawl poll. Sakurai chose Zero Suit Samus because Metroid: Zero Mission recently came out and chose Lucas because Mother 3 was coming out soon in Japan. Sakurai chose Ike because the first Fire Emblem game on the Gamecube was coming out soon and chose Lucario to promote the upcoming 4th gen games. Sakurai chose Toon Link to replace Young Link because Toon Link is the star of The Wind Waker, a popular Zelda game in Japan. Sakurai chose Olimar because Pikmin was the only new character-driven franchise that was very successful on the Gamecube. Sakurai chose R.O.B. as the new wild card.
As for the other possible newcomers for SSB4, Sakurai could always look into the top choices on the Brawl poll. Geno (though I doubt it), Krystal, King K. Rool, Dixie Kong, Takamaru, Mega Man, Ridley (maybe), Isaac (but the new protagonist could be playable in his place), and Little Mac are some of the most popular choices on the Brawl poll who are not playable in Brawl.