Tried to post this to a blog, but it keeps coming up with a validation error, and it's just not worth fixing, here's my speculation:
I'm at work, so I can't play anything, so here's my attempt at a 55 to 60 characters SSBIV prediction as suggested through the thread. I didn't make it to that many, because I honestly can't see any other Characters getting in besides the ones mentioned, and even then, I just added a lot of these to fluff the roster out. Newcomers in italics, anything I feel needed an explanation should have one.
48/53
Mario Series
Mario
Luigi
Peach
Bowser
Bowser Jr. - I think Bowser Jr. Has great potential for a moveset, and won't appear, save as a faster Bowser clone, and even then, it'd be unlikely. Regardless, I'd still play as the guy.
yoshi series
ONLY YOSHI. - WHAT OTHER CHARACTERS ARE THERE EVEN? KAMEK? THAT ONE ENEMY? WHO CARES?
Wario Series
Wario - I think Wario is a safe bet to return, but his cronies just don't have the history or awareness for a moveset.
DK series
Donkey Kong
Diddy Kong
Dixie Kong - I only put Dixie because I remember she was slated for Brawl, and I think that helps her chances.
King K Rool - IF the Kong series gets 4 reps, I think we are guaranteed a villain. we would've gotten him earlier, but I don't think a moveset was even thought of for him.
Zelda Series
Link
Toon Link - It's possible Toon Link won't appear, since I haven't heard word of a new handheld game from the Zelda series.
Zelda/Sheik
Ganondorf
Skyward Sword's Flamboyant villain - Incredibly dubious on this one. I'm only suggesting it because I want more Zelda, but I'd honestly be happier with Majora or Stalfos Knight, neither of whom have a prayer.
Metroid Series
Samus
Zero Suit Samus
Ridley - Ridley got a lot more characterization in Other M, good or bad. Whereas that game REALLY hurt Samus, I think it helped Ridley's chances in Smash Bros out a little, if nothing else. Also, he's the only character that appears in the Melee intro that hasn't been made a playable character yet.
Adam - I hope not, but I think he's honestly more likely then Ridley.
Starfox Series - I'm hoping we stay at these 3, I honestly don't see the need to drag any more characters into it.
Fox
Falco
Wolf
Kirby Series - Think of another Kirby character that isn't a minion or a final boss that's appeared in multiple games, and you'll realize, there's no where else to go with this roster.
Kirby
Kind Dedede
Metaknight
Mother Series
Lucas - Lucas actually runs the risk of not returning, since his moveset wasn't really unique (a missed oppurtunity, to be sure) and his game is no longer current. I do hope he does though.
Ness
Kid Icarus Series
Pit
Magnus - Sakurai worked on the new Kid Icarus game, and mentioned getting ideas for Brawl, a quick wiki of the major characters makes me think Magnus is the most likely from that roster to make it in, though I think Black Pit is also a possibility (but will probably just be a Pit outfit)
Eggplant Wizard - Yeah, this one's a joke.
Fire Emblem Series
Marth
"Hero of the Shadow"/Ike - I think this really depends on if they choose to go with the handheld protagonist or Ike, and if they decide that the "Hero of the Shadow" is the protagonist and not Marth. I suspect the moveset won't vary to greatly though.
The New Fire Emblem's Main Villain/The Black Knight - Depends on which of the above is picked, if both are picked, neither of these guys has a prayer. Either way, I don't think FE will get more then 2 reps unless the roster increases greatly.
F-Zero
Captain Falcon
Pokemon Series
Pokemon Trainer
Pikachu
Jigglypuff
Mewtwo - Only because he was supposed to make it into Brawl.
Zoroark - I think this will be a our current gen Pokemon Rep, taking the place of Pichu and Lucario, i doubt his moveset will change much from Luke's, which is kind of a shame.
Ditto Not actually a possibility, I suspect. I just think Ditto would be a cool concept, he would be your opponent's char, and in the case of multiple characters, load a different char each time you died.
Pikmin Series
Olimar
Golden Sun Series
Isaac - Dunno how well Isaac would fit, but if the Roster is expanding, I suspect he's got a pretty good shot.
Retro Series
Ice Climbers
ROB
Mr. Game and Watch
Balloon Fighter - I think Balloon Fighter is the most likely Retro addition, though I'm not sure we'll receive any.
Mach Rider - Doesn't actually have a chance. It's a shame too.
Other Nintendo
Mii - I think Mii is much more likely this time-around. During the Wii, we got no Mii characters in non-sports games made by Nintendo. However, with the WiiU, it looks like they'll be playable in NSMB (which is great) so maybe the WiiU will see some of those Miis getting the playability they deserve.
Third Party Characters - I think in reality we'd only get 3 max of these.
Sonic
Tails - Unlikely, but if Sega gets another rep, they won't "waste" it with a GOOD character!
Snake
Simon Belmont - I think Simon has the highest chance of being Konami's addtl rep, which is something of a shame, because Bomberman would be more interesting, but less of a shame then any other character, since his whip moves could make him pretty unique.
Bomberman - I would love to see this guy, I don't think he has a prayer, but he's one of my favorite characters.
Megaman - Megaman has a higher chance this time round, because Nintendo knows that his creator would allow it.