I wonder what the world breakdown would be. Here are my thoughts for what it would be given the current Brawl characters + villains and characters I think will get in. I'm thinking essentially playing four "good guys" who at least think they're fighting a "bad guy" for each world story arc.
Mushroom Kingdom
Mario, Luigi, Peach, Yoshi
vs.
Bowser
The Wilderness
DK, Diddy, Ice Climbers, Sonic
vs.
K.Rool
Arcane Kingdom
Link, Zelda, Ike, Marth
vs.
Ganondorf
The Great Capitol
Samus/Z-Suit Samus, Captain Falcon, ROB, Solid Snake
vs.
Ridley
Dream Land
Kirby, Meta Knight, Ness, Lucas
vs.
King Dedede
Pokemon Country
Pikachu, Jigglypuff, Lucario, Pokemon Trainer
vs.
Mewtwo (or other pokemonish villain, I'm sure some pokemon's gonna get in)
Orbital Array
Star Fox, Falco, Captain Olimar,
vs.
Star Wolf
The Lost Islands
Game&Watch, Pit, Toon Link
vs.
Wario
*After these worlds have been completed, it unlocks the story arcs where you go through the "villains" story.
Desolate Wasteland Front
Bowser, King Dedede, K.Rool, Wario
vs.
Non-playable villain or Secret Character
Desolate Wasteland Back
Ganondorf, Ridley, Mewtwo, Star Wolf
vs.
Another non-playable villain or Secret Character
FINALLY, after completing all the "good guy" arcs and these two "bad guy" arcs, the total story comes together with all of the playable characters to defeat the final area and boss. Fin.