Okay, so... here's a little speculation....
If all of the characters on the dojo aside from snake and sonic are the starting roster, then how many unlockables will there be? Well, let's look at Melee first. It had a total of 25 characters, 26 if you include sheik separately. 11 of those were unlockable. Just shy of half of the total characters were unlockable.
Okay, now Brawl's starting roster is just shy of the total number of characters from Melee. in other words the number of starters has almost doubled (give or take about 5). If those numbers extend to the unlockables, then let's just double the unlockables from Melee.
That's 22 unlockable characters. Now let's scale it back to 20 just for posterity. How many characters do we know/have-reasonably-strong-evidence-for that aren't amongst the starters? Snake and Sonic for sure (2). Ness, Jigglypuff and Lucario are almost certain (3). Game & Watch, Mewtwo, Marth, Captain Falcon, Luigi and Ganondorf to round out characters that had unique movesets or are likely to return(6). And even if you really wanted to, you could throw in Falco (1).
That's 12 characters total that have some moderately strong basis for appearing or re-appearing in Brawl. That leaves 8! (FREAKING 8!!!!!!) total empty slots for unlockable characters. Heck even if you threw in ALL of Melees characters you've still got 4! But for the rest of this discussion I'll consider it as 8.
Feel Free to fill in who you think are likely candidates for the slots, really this doesn't need to turn into a thread discussing which characters we'd like to see in Brawl, but since I've never done so, I'll throw in my opinion.
1. Krystal - She's got a real strong chance, IMO
2. Geno - Hotly debated and all, I like the guy and SMRPG is an important part of my early gaming experience.
3. Megaman - I don't like his shaky chances, but if there is any other 3rd party character more deserving to get in, I'd doubt it.
4. Dr. Robotnick - Don't give me eggman crap, I'm oldschool man. lol. Every Hero needs a villain.
5. Ridley - In the vein of villains, I think Ridley has the best shot of inclusion when compared to Dark Samus...
6. Dark Samus - ...But i wouldn't count Dark Samus out. She doesn't have to be a clone as the inherent corruption and light/dark powers could give rise to an awesome unique movest.
7. Black Shadow - Captain Falcon's Nemesis
8. Isaac - Hero from Golden Sun (Golden Sun's Composer is working on the game, giving extra boost to his chances)
The Masked Man - Earthbound/Mother villain.
Eggplant Wizard - Kid Icarus Villain
Cell-Shaded Link - An ever unpopular version of Link amongst Brawl fans, I think he not only has a shot, but has enough possible variety for a moveset to make him anything but a clone.
Bowser Jr. - Unlikely as a character, IMO.
Ray - Custom Robo series
Simon Belmont - Unlikely chances, but an awesome idea, IMO.
DeMille - Similarly unlikely, but another personal fav.
Viewitful Joe - Again, unlikely, but fun idea.
Of course this is just if my calculations are anything remotely resembling correctness. I could be horribly wrong and we may know all of the unlockable characters already (luigi, jigglypuff, Lucario, Ness, Snake, Sonic). But when I think of how sure Sakurai is when he says that the number of playable characters will blow us away, and about how the numbers game I just played is so good-looking, I can't help but be confident that we'll be seeing a LOT of unlockable characters.
What do you all think?