I'll post my latest predictions here anyway. Everyone from Brawl returns, plus
First Appearance: Pokémon Red/Green (GB, February 1996)
Latest Appearance: Pokémon Black 2/White 2 (DS, June 2012)
By far, Mewtwo is the most popular of the characters who missed the jump from Melee to Brawl. It also bugs me that he's the only non-clone character to have not made the cut. Actually, he very nearly made it into Brawl, but the last minute inclusion of Sonic is most likely what did him in. Despite the glory days of Red and Blue having passed, Mewtwo remains a recognizable and iconic Pokémon, and it's never a bad thing having more villains on the roster.
First Appearance: Wii Sports (Wii, December 2006)
Latest Appearance: Mario Kart 7 (3DS, December 2011)
In July 2005, when Sakurai finalized the Brawl planning document, Miis were over a year away from introduction. How was he to know how prevalent they would become? For better or for worse (
worse), Miis are the closest thing Nintendo has had to a solid new franchise this past console generation.
These things are
EVERYWHERE.
First Appearance: Animal Crossing (N64, April 2001)
Latest Appearance: Animal Crossing 3D (3DS, 2012)
In the February 29, 2008 issue of Famitsu, Sakurai said that he was thinking about including the playable character from Animal Crossing (using a shovel and net), but in the end he had to prioritize and include other characters who more closely fit the "image" of Super Smash Bros. With those priority characters out of the way, it's probable that the Animal Crossing character has reemerged at the top of Sakurai's list of candidates. It's also worth noting that since the completion of the Brawl planning document, Animal Crossing has gained a feature film, a Wii game, and an upcoming 3DS game. It's one of Nintendo's biggest franchises.
First Appearance: Punch-Out!! (NES, November 1987)
Latest Appearance: Punch-Out!! (Wii, July 2009)
Some people were surprised that Little Mac wasn't included in Brawl, but now that Punch-Out!! has been "revived" with a new Wii game, there's a good chance that we'll see him in SSB4. He's a fairly iconic Nintendo character (especially in the West).
First Appearance: Metroid (FDS, August 1986)
Latest Appearance: Metroid: Other M (Wii, September 2010)
By this point, Sakurai probably realizes that there's a lot of demand for Ridley as a playable character. He's also made two major appearances since Brawl. Oh, and he isn't too big.
First Appearance: Paper Mario (N64, August 2000)
Latest Appearance: Paper Mario 3D (3DS, 2012)
There are three Paper Mario games with a fourth in development, and the series has become Intelligent Systems' flagship franchise next to Fire Emblem. He'd make for a fun rival to Mr. Game & Watch.
First Appearance: Kid Icarus (FDS, December 1986)
Latest Appearance: Kid Icarus: Uprising (3DS, March 2012)
Sakurai just finished one of his most ambitious works yet, a revival of the classic Kid Icarus. Next to Pit, Palutena is the most important and most recurring character in the series. For that matter, Kid Icarus is actually called "Palutena's Mirror" in Japan. She's the Princess Zelda of the franchise.
First Appearance: Pokémon Black/White (DS, September 2010)
Latest Appearance: Pokémon Black 2/White 2 (DS, June 2012)
It's common sense that there's going to be a new Pokémon character, and generation six is a long ways off. Seriously, Sakurai doesn't plan that far ahead with his rosters; Brawl's was finalized on July 7, 2005, and the game didn't come out until 2008. Ultimately, it comes down to Zoroark and Victini. Historically, all of the cutesy Victini-like legendaries have been relegated to Poké Ball summons, so who knows. Zoroark is essentially the Lucario of Black and White.
Only Appearance: Fire Emblem: Awakening (3DS, April 2012)
Between Roy and Ike, it's obvious that Sakurai likes to represent the latest Fire Emblem games with a playable character. With DLC on tap for Awakening, not to mention any future projects in the Intelligent Systems pipeline, it's unlikely that we'll see the next installment in the series until 2014-2015. The Zoroark entry explains Sakurai's commitment to an early roster selection.
First Appearance: The Mysterious Murasame Castle (FDS, April 1986)
Latest Appearance: Samurai Warriors 3 (Wii, December 2009)
Melee had Ice Climbers and Brawl had Pit; who's going to be SSB4's NES revival character? Takamaru has experienced a resurgence of sorts over the last few years with appearances in Captain Rainbow and Samurai Warriors 3, and just as Pit is the "sister" character to Samus, Takamaru is the "sister" character to Link. Even though he's entirely unknown in the West, Murasame Castle is well-regarded in Japan.
First Appearance: Sheriff (AC, 1979)
Latest Appearance: WarioWare, Inc.: Mega Microgame$! (GBA, March 2003)
In addition to the retro NES characters, Sakurai has included an unexpected "historical" character in each installment since Melee (Mr. Game & Watch and R.O.B.), and who's more historic than Nintendo's first video game character? Not only that, but Sheriff is also the first character designed by Shigeru Miyamoto! He's the Oswald the Lucky Rabbit to Mario's Mickey Mouse. Ideally, he'd be given a 3D model based off the old advertisement/concept art and not be sprite-based. He'd be a lighthearted "joke" character who uses a pistol, lasso, and dynamite in comical, over-the-top ways. He'd even enter the fight on his horse.
First Appearance: Mega Man (NES, December 1987)
Latest Appearance: Mega Man 10 (WiiWare, March 2010)
Assuming Snake and Sonic come back, Mega Man seems to be the favorite third-party newcomer. In terms of major Japanese third-party publishers, there's Square Enix, Capcom, and Namco Bandai left. Slime and/or Pac-Man are possible, but they don't have nearly the fan demand or move set potential that Mega Man does.
So yeah, including transformations, that's 51 playable characters in all. Sorry for such a long post. Of course, come E3, I'll probably be doing my list all over again, but this is all based on current knowledge and Sakurai's past roster selections.