As for Mushroom Kingdoms: I would think that this is another name for Nintendo's Online world.. back in the days of the "StarCube" someone had an interesting Idea that Nintendo's online community would be named and created as a true community with different places (bearing Nintendo Trademark names) serving as the forum, chat room, gaming area, etc. [i.e. Hyrule Castle is the chat room, blah blah blah]
Now, what I would like to see is an online Zelda game, maybe not featuring Link, but instead you could be of the different races (Goron, Gerudo, Zora, Sheikah, Kokiri [though Kokiri can't leave the forest.. oh well they could find away around that], or Hylian)
And maybe the game could take place after Zelda: The Adventure of Link, when Hyrule starts rebuilding, and the Kokiri could be more like wood elves, the sheikah could be like ninjas, and the Gerudo could be awesome fighters (like the Aiel in Wheel of Time)..
If Nintendo is kind to us, then this announcement will be Kid Icarus, except If it is then I hope that it has lots of wicked fights or powerups... and eventually you could have those flying stages and be strapped with crazy Magick out the yin-yang