I want to see:
Mega Man
Little Mac
King Hippo
alt Pokemon Trainers (ie. Choose Gold and you get Totodile, Bayleaf, and Typhlosion, choose Ruby and you get Mudkip, Grovyle, and Blayziken, etc.)
Captain N
Fawful
Dr. Wily
Ameterasu
there's probably more, but... eh...
way I see it, since they added Sonic and Snke with the last one, it's not so much Nintendo characters as it is Nintendo exclusives (Snake started on the NES, Sonic... has no excuse, why is he there?)
as for stages... maybe these:
a Pokemon battle screen (screen from the game, pokemon are sent out, attacks do damage)
the train from Spirit Tracks
something for Mega Man (flying platforms, bouncing enemies, badass remixes)
we also need a soundtrack. as in physical CD, iTunes, or free download from the Nintendo site, we NEED this music. Nintendo, listen, if you release a soundtrack for Brawl... YOU WILL MAKE SO. MUCH. FUCKING. MONEY. you will literally be able to buy people. you will be so rich you will be able to choose who lives and who dies. and that's why Nintendo should release a Brawl soundtrack