Okay, here's my roster in text format:
MARIO FRANCHISE: Mario, Luigi, Peach, Bowser, Bowser Jr.
Jr. has a sizable fanbase (that actually wants him playable, unlike the Japanese Toad fanbase) and can be given either a unique or cloned moveset...a flexible character in every sense of the word.
DK FRANCHISE: Donkey Kong, Diddy Kong, King K. Rool
Moving right along...
LEGEND OF ZELDA FRANCHISE: Link, Zelda/Sheik, Ganondorf, Child/Toon/Young/Potato Link
I don't believe an unpopular or one-shot will be shoe-horned in for the sake of filling slots. Although Ganondorf and Toon Link might receive some moveset changes, this series remains rather stagnant.
METROID FRANCHISE: Samus/ZSS, Ridley
Obvious selection is obvious.
YOSHI FRANCHISE: Yoshi
. . .
KIRBY FRANCHISE: Kirby, Meta Knight, King Dedede
Bandana Dee is tossed around for the same reasons as Waluigi: "He'd complete the RtD four!!" He has an outside shot, but I'll believe his likelihood when I see him confirmed.
STAR FOX FRANCHISE: Fox, Falco, (Wolf OR Krystal)
I want to believe that Wolf is going to stay, but I really doubt the series will receive four characters. Of course people will say a character they don't want to see leave has no chance of leaving, but when you consider that Wolf was such a last-minute addition to Brawl and thus might not be that high on the priority list of veterans to carry over, the chance of him getting clipped (possibly for a moderately "popular" female) still exists.
POKEMON FRANCHISE: Pikachu, Jigglypuff, Pokemon Trainer, Mewtwo, Zoroark
I've concluded that Mewtwo will be brought back over Lucario, since he has more importance to the series as a whole than the newcomer who was selected to represent its latest generation at Brawl's time. It's unrealistic to think that Pokemon will not receive a newcomer when Pokemon Company is as involved as it is, having the opportunity to promote a new Pocket Monster. Though Mewtwo, Lucario and Zoroark can coexist, I definitely don't expect Pokemon to receive EIGHT characters in a game that "has reached its limit" balance-wise.
MOTHER FRANCHISE: Ness, Lucas
Lucas -could- get cut, but there aren't any Mother characters of his caliber to fill his shoes (Porky would be awesome, but an impossibility due to it being "probably impossible" for Sakurai to animate when matters like "fleshing out" the SSE are far more important).
F-ZERO FRANCHISE: C. Falcon, S. Goroh
I'll flip a coin here and say Goroh gets in, hopefully to help promote the first F-Zero game in almost a decade.
FIRE EMBLEM FRANCHISE: Marth, (Ike OR Roy), Krom
Likewise, a new Fire Emblem representative has a strong chance, while Roy is apparently popular worldwide because of how superb his character is...pah. I would hate to see him edge Ike out, but I don't think the series' number of slots will double overnight either, and Ike is the most expendable character out of him, Marth and Krom.
RETRO CHARACTERS: Ice Climbers, Mr. Game and Watch, R.O.B., (new character, Takamaru as placeholder)
Only including popular characters "doesn't excite" Sakurai, and he's "the luckiest man in the world" because he gets to revive retro characters. Takamaru seems most likely to get the nod at this point, though any reasonably old one-shot, like Mach Rider or Balloon Fighter, could do.
WARIO FRANCHISE: Wario
. . .
KID ICARUS FRANCHISE: Pit, Palutena
Though I despise her being labeled a massively popular shoo-in mere months after she even became a viable choice (taking over for Medusa, if you will), she's the go-to character if Sakurai doesn't become modest, which he won't, because like with Kirby for Brawl, he'll consider this the last chance he has to work with the character he helped design.
NEW SERIES CHARACTERS: Little Mac, Saki
Little Mac is a very, very obvious choice. Saki is a somewhat less obvious, but rather viable selection as well. Additional slots may go to any of Isaac, Shulk, Muddy Mole, Starfy, etc., but I think these two have the most priority, and space is tight.
THIRD PARTY CHARACTERS: Snake, Sonic, Megaman
After how popular Snake and Sonic became, I think Sakurai will at least attempt to negotiate with Konami and Sega for their returns. The companies would never say "No, maybe they shouldn't appear this time," so them getting clipped would be Sakurai's decision alone, and considering he added Sonic so late in Brawl because of the predicted backlash his absence would cause, I think he's smart enough to keep them in to avoid even more of an uproar. Megaman is pretty much universally agreed upon as the third third-party character, even by the Capcom execs that seemingly ignore him, and Sakurai has mentioned the possibility of Capcom characters. Pacman could become playable as well, but I want to wait and see how much influence Namco has on the project before making that call.
This brings my total to 50 characters, counting transformations. If Wolf/Ike are kept playable, that leaves in 40 veterans and 10 newcomers, while if one or both are not, the make-up changes to 38/39 veterans and 11/12 newcomers.
EDIT: Yes, the interview supports my confidence in Snake/Sonic, and confirms my doubt in less important, filler third party characters (Slime, Layton, Travis Touchdown, Tales characters).