ok first off w00t, peach confirmed although that gives away monday's update xP
Secondly, (just to throw it out there) i honestly think that you'd have to play as different characters in each level throughout the story mode, kinda like Event Matches, only all of them linked together by one story. Like the first level would be Mario/Kirby/Peach/Zelda fighting off the robot-thingys, then the plot somehow links together someone else (Link and Yoshi for example) then you play as one (with comp)/Both (in a hopefully incuded co-op) in whatever level they'll have. Then repeat and somehow have all the characters be involved and come together in the end where (somehow) you'll be able to choose a character and beat the last boss, get the character specific ending and repeat with other characters.
Although it brings up the question of whether on not the adventure mode gets longer when you unlock characters. I.E. Snake is confirmed for an unlockable, but will he have a part in the adventure mode after you unlock him?
Also, Space Emissary is a subtitle, meaning HOPEFULLY (and doubtful) there would be other parts to adventure mode.
Thats just my two cents.
PS, what's an emissary? XDDD (I'm thinking like an organization, but just to clarify)