They will probably leave some characters hidden until the game launches. There are several Brawl characters still missing. 

Cool to see a bit more information about the Elite Four stage as well. While in the Dragon room, a bunch of Dragon Pokemon (Garchomp, Hydreigon, Axew, and Dragonite) are watching.

Nintendo appears to have 8 more characters (probably all returning) to reveal. 

Brawl characters still missing: 

Wario

Ganondorf

Ice Climbers

ROB

Falco

Wolf

Pokemon Trainer (Charizard is separate, so he may not return)

Jigglypuff

Ness

Lucas

Mr. G&W

Snake

11 Brawl characters, 8 slots. Who gets the axe?

Oh great, another Fire Emblem character!!!

Snake, Trainer, Lucas, and Wolf are the strongest candidates in my mind.

Nice catch Iamawesome!

If there are 8 characters left to be named, my guess it's an even split between vets and newcomers. For me, I think Jigglypuff and Ness (as the originals) will return along with G&W and Ice Climbers. That'll leave four more characters to be revealed.

Just my opinion obviously.

Where's Ness T_T

Ness HAS to be in Smash. Just like Jigglypuff. They're the last two of the originals, they'll be in the game. If one WAS to be left out, it would be Jigglypuff, as Pokémon has numerous representatives. Ness represents Earthbound and thus WILL be in the game. Sakurai reads the fan comments, he knows how much we love Ness and Captain Falcon, that's why he put CF in the Lucina/Robin reveal.

Srry guys but that doesn't mean anything (about the blank boxes). Ita been that way since the beginning when the site opened up. Pokemon does the same thing. The game can have just 3 more characters,or it can have 10 more. Once those boxes are filled up, another blank row will show up, that doesn't mean will have 53 characters.

Also i think Trucks pointed out that those boxes appear differently sometimes depending on how your searching it up!

Same place as Captain Falcon and Metaknight were XD

To be honest though, I think 8 more characters would be a nice fit. Four more veterans, and four more newcomers. Two more videos announcing four characters. Not saying that'll happen, but I think it'd be nice.