More updates to the confirmed page as I looked over old news articles.
Of course, one of the biggest things about Brawl is that it's the first game in the series to let in some characters from non-Nintendo games. Snake started the trend, and it'll most likely continue. But who will get the coveted 2 (possible) remaining slots for 3rd party characters? Here's some possibilities, and they're likely hood.
Who?: Former console maker and Nintendo rival, Sega has since gone 3rd party and has been a big supporter of their former competitor.
Relationship?: As I said, Sega has been a really big supporter of Nintendo's home and portable consoles. Most recently, they finally did the impossible and threw in their signature character, guess who!, in a game with former mascot rival Mario. Considering how much people want guess who! in Smash Bros. as well, they seem to be the most likely supporter.
Have they said anything about it?: Yep. Not only have certain former and current Sega developers (including guess who? creator Yuji Naka) have said they'd like to see a character, namely guess who?, in Brawl. After the announcement of Mario & guess who? at the Olympic Games, they remained tight-lipped about the possibility, although some Nintendo reps have said both "We can't comment on that right now." and "That's not on the cards right now."
Character Possibilities: There's only one character who can possibly be in the game, and that's only because some people would go bonkers if someone else from their roster was there instead. You already know who that is.
2. Hudson Soft
Who?: One of the most prolific developers of the 8 and 16-bit eras, Hudson has supplied the world with classic titles like Bomberman, Bonk's Adventure, and Adventure Island. They continue on with the Bomberman games to this day.
Relationship?: Hudson makes quite a few games for Nintendo, including developing the entire Mario Party series. They are one of the big supporters for the Virtual Console, supplying the Turbografx-16 games.
Have they said anything about it?: Not really, although a post on the official American Hudson website did ask it's readers if they liked the idea of Bomberman or Bonk appearing in the game.
Character Possibilities: Bomberman (most likely, being their most popular character), Bonk (former mascot, although has only appeared a few times on Nintendo consoles).
Who?: The two most famous RPG producers in the world, now combined into the the one most famous RPG producer in the world. The makers of games like Final Fantasy and Dragon Quest/Dragon Warrior.
Relationship?: After staying away from the N64, the companies in recent times have become quite enamored with Nintendo's portables. Now they're back to being fairly close, even collaborating on several projects and recently having many crossovers (Mario Hoops with FF characters and the upcoming Itadaki Street DS with Mario and Dragon Quest characters.)
Have they said anything about it?: Jack, except when Hideo Kojima asked FF and Kingdom Hearts creator Tetsuya Nomura what he thought of the possibility, to which he replied to the effect of "They haven't asked me."
Character Possibilities: With each new FF and DQ having a new cast, specific characters seem unlikely, and more widely recognized series mascots like Mog, Chocobo, Slime, or one of the Final Fantasy I character classes have been suggested to be possibilities. It's been suggested that Sora, the main character from Kingdom Hearts, could appear, but corporate ties with Disney and the fact that he has only appeared in one Nintendo game stifle his chances. Older characters (including those from favorites like Chrono Trigger) are possibilities, but might be passed up due to unfamiliarity among the current era.
Who?: One of the biggest, most diverse publishers in the game industry. They are famous for creating everything from Street Fighter, to Mega Man, to Resident Evil, and then some. They also have a long history of throwing all their characters into big fighting competitions.
Relationship?: Not incredibly devoted to Nintendo, but they produce plenty of games (including most of the Mega Man games, and several Resident Evil titles) for their systems. It is also to be noted that Street Fighter creator Yoshiki Okamoto is good friends with Shigeru Miyamoto, and helped make Legend of Zelda titles on the Gameboy Colour, Gameboy Advance, and Gamecube.
Have they said anything about it?: Nada, unless you count an anecdote told a while ago by a poster on the 1Up EGM board, in which someone asked Mega Man creator Keiji Inafune (who was on a mall tour promoting his Xbox360 games Dead Rising and Lost Planet) whether he thought his prized creation should be in Brawl. He replied "Wouldn't that be a nice surprise?"
Character Possibilities: The top one is, of course, Mega Man (who had comments asking for his appearance printed on Toukouken!). Other possibilities include Street Fighter mainstay Ryu, or a main character from one of the Resident Evil titles. Characters from cult hits like Phoenix Wright: Ace Attorney have also been discussed, although usually not seriously.
Who?: Probably most well-recognized as the people who brought us Pac-Man, Namco was an arcade dynamo back in the 80s and 90s, producing hits like Galaga, Dig Dug, and Pole Position. In modern times, they seem more heavily foused on popular fighting games Tekken and Soul Calibur.
Relationship?: Reasonablly well-off, Namco puts many of it's games on Nintendo consoles. The appearance of Link in Soul Calibur 2 four years ago has made many people demand that Namco 'return the favour'. Shigeru Miyamoto is also a fan of Pac-Man (which spawned the game Pac-Man Vs. on the Gamecube.)
Have they said anything about it?: Nope. Not a peep.
Character Possibilities: Pac-Man (Shiggy likes him), Klonoa (a cult character from the titular series), a character from Soul Calibur (Yoshimitsu, who is also a character in Tekken, is the most popular choice), a character from Tales of Symphonia (although there is an entire games series behind that title), or even the guy from Dig Dug and his son from Mr. Driler.
Who?: French developer who started off as the guys who made Rayman, they have since created a few more popular series, including the new Prince of Persia series and Splinter Cell. They were adamant supporters of the Wii when the first games were showed off at E3 2006.
Relationship?: As already mentioned, they are big fans of the Wii and have been one of the biggest third party supporters on the system.
Have they said anything about it?: Not directly, although there was that rumor...(check the 'Everything Confirmed' page for that).
Character Possibilities: The most likely is Rayman, but others that have been suggested include the Prince (from Prince of Persia, of course), Sam Fisher of Splinter Cell fame, and Jade from the cult-favorite Beyond Good & Evil.