I know that Super Meat Boy is all about the indie, as is abundantly clear from the characters included, but I think it would have been fugging amazing if they were somehow able to negotiate with EA and Ubisoft to include Ezio/Altair and Faith (Mirror's Edge), seeing as how they're at least tangentially related to running and traversal. They obviously don't fit at all, but if they were incredibly stylized to look appropriate I think that would be super cool.  
 
But enough about me, who would you like to see?
 
PS Can someone draw up what they would look like in SMB fashion? I'm shit at art, so I call upon thee, GB community.

Crash Bandicoot should be in every game ever.
 
EDIT: for reals though, I'm glad this game is all-indie.

The cat from syobon action

Bowser

" I like that it's all indie characters and would not want that to change. That being said, I would like to see the kid from Limbo. "

Viridian from VVVVVV, or Guy(I don't know his name) from And Yet It Moves- flipping gravity would be pretty amazing in this game as one of the PC exclusive characters. Would definitely need to be a character requiring a lot of bandages to unlock. I don't know about Wii exclusives, I am not too familiar with WiiWare titles besides what they read on the Bombcast and Bit.Trip.
 
Kinda wish 'Splosion Man made it into the xbla version too. His "'Splode Jump" could be a triple jump and could kill small enemies (like the purple things in world 2 or the maggots in world 5) and knock away missiles and such. I guess his disadvantage could be that his wall jump is at an inferior angle, requiring a second midair jump to gain any height while jumping on the same wall and 'sploding on an enemy or projectile could kill his momentum, leaving him vulnerable for a second to get hit or fall to his death.
 
If I absolutely had to pick a non-indie character (though I don't know that I would like to see that) I would pick Chell from Portal. I know world 4 does have a few of those portals already but being able to create your own would be incredibly broken. She would have to not be able to wall jump and just use portal momentum or something to keep it balanced. Would be an interesting way to play the game.

" I think the Robot from Machinarium shoulda been in there(name escapes me atm) "

" @ImmortalSaiyan said:

" I like that it's all indie characters and would not want that to change. That being said, I would like to see the kid from Limbo. "

Naw, F that! So he can die every 2 minutes?! That kid was a punk! A worthless punk! He could barely even jump. "

how cool would it be to have Yoshi in there? he could have his flutter move to have a different jump behavior and sticking to walls could be justified by him using his tongue :)
 
also, the guy from VVVVVV ...including the gravity controls LOL that would be hardcore :D (or is he already in there?)

the ball from the ball. i don't know how it would work but it would.

Captain Viridian (VVVVVV) will be in the PC version and they're won't be any DLC characters for the Xbox version, just "map packs" basically

Splosion Man

" Captain Viridian (VVVVVV) will be in the PC version and they're won't be any DLC characters for the Xbox version, just "map packs" basically "

Josef from Machinarium!

I wanted Captain Vi ridian and he will be in the PC version. So I may have to buy that one too.