Reading through all this, i had Kirbys Funk Pak/Super Star in my head, with the great cave level thing, where you gather treasure. Just reading through peoples ideas gave me an idea (and a first post i think hah?)
What if they produced something like The Great Cave Offensive section of Kirbys Fun Pak/Super Star?
For those that dont know it:
Its basically one huge level cut down into (i think) 4-5 different areas. One being an open outdoor area, another being a castle area, and so on. Basically just different themed areas. Now these different themed areas in total have 60 treasures for the player to find - and most likely if its your first run through, you won't collect them all. But the thing is, you could run through all the themed lands (they appear one after another) without collecting ANY treasure. Therefore it could be a short game, depending on your hunting tactics etc. Every themed area is divided by a boss to add to the difficulty.
So my idea for Brawl is:
Have one huge level, like TGCO (The Great Cave Offensive). Now this is where my idea splits. There will be two game modes, depending on if you're a "hero/heroine" character or a "villain character". If you're a hero, the world can be split up into however many themed world... lets say 6 for example. You can go through the worlds at your own pace collecting the "treasure" (in Brawl it could be trophies or something to unlock new stuff - i dont know, whatever is collectable in this game i suppose), or rush through to the end just fighting the bosses etc. So i'll give an example of how it could run
(presuming you're a hero character)
>Starting point
>First Level (A level from one of Bowsers Castles in the original Mario Bros. but spruced up)
>Boss : Bowser
>Second Level (Starts off from a section of Hyrule field, and the ending point is inside Hyrule Castle, however it is ALL in Twilight, where you fight...)
>Boss : Ganondorf
>Third Level (Starts off from one end of the Halberd or whatever Metaknights ship is called heh, and you must reach the bridge)
>Boss : Metaknight
>Fourth Level (Lets say its Corneria? But you start on the ground or on top of the buildings, must make your way ontop of a moving Great Fox, followed by on top off the Star Fox and Star Wolf team ships to fight...)
>Boss : Wolf (yeah i struggled for thinking of villains so im just making this up, im not saying hes confirmed in it hah - just to help along my idea
)
>Fifth Level (F-Zero race track, where you must jump from car to car till they finish the race - bearing in mind they'll be trying to destroy each other and if you get in the way it leads to high % damage?)
>Boss : Samurai Goroh (again to help my idea along)
>Sixth Level (Pokémon themed level. Not quite sure about this. Presuming the characters in it, you must fight/force your way constant moving rubble which is being controlled Psychicly, up untill you get into the stadium from the first movie to fight...)
>Boss : Mewtwo (if hes in it)
Thats just speculation ^^ - Just an idea of the flow, give or take levels, and obviously add more to go with most characters and what not. However this order/the scenarios would change as i said depending on if you're a hero/heroine character or a villain. The above would be for Heros and heroines. but now for an example for Villains.
>First level (The first level from original Mario Bros. but spruced up)
>Boss : Mario/Luigi or Peach?
>Second Level (As if based before it was Twilight, have it start from a desert-like setting part of Hyrule, going all the way upto Hyrule Castle - but in the light world (as if twilight hasn't struck Hyrule yet etc ))
>Boss : Link/Zelda/any other hero/heroine LoZ character
>Third Level (A typcal styled level from Kirby such as Green Greens? with all teh usual suspected enemies and what not, finally winding up to a possible Whispy Woods - either alive and blowing wind or as if he's just been beaten by...)
>Boss : Kirby
>Fourth Level (Starts off on top of Corneria about Venom, followed by jumping from Arwings/Star Wolf teams ships, to finally reach the surface of Venom where infront of Andross.. perhaps? you must take on...)
>Boss : Fox/Falco/Heroic Star Fox character
>Fifth Level (Very Similar to the other F-Zero level. Maybe same aspect but entirely different course?)
>Boss : Captain Falcon
>Sixth Level (Obviously this level would be following as if Mewtwo was in it, so perhaps it could start at Victory Road, make your way through that, then climbing the top of the Indigo Plateau (or even Johto, Hoenn or maybe Sinnoh league stadium, it could be any! Jumping inside the stadium to face...))
>Boss : Pikachu/Pichu/Jigglypuff - depending on if teh last two are in it, or if there are newer Pokémon there such as Lucario
Sorry for the VERY long first post
but... i think it could be a working idea. You could run freely through the levels as fast as you do in Melee Adventure Mode, or really take yoru time to discover the themed levels and find the "treasures" or possible trophies or whatever you collect in them
Let me know what you think *prepares extinguisher for when im flamed!* XD
more fun and all that good stuff.....sorry the fighting is cool but i'm a girl and i'm all for the fun in video games. but i do like the occasional fighting and battle stuff 
