^^Pulled solely from my ass (don't touch it):
I love a 2D Mario adventure as much as the next guy but do we really need one so soon? I mean, look at Mario 1, 2, 3, World, New, and New Wii. The first ones, up until NSMB were all very different from each other. When you look at NSMB Wii, it looks and feels a lot like the DS version with a tacked on, frustrating multi-player gimmick. That's not how I want my beloved series to evolve.
We hold the "real" Mario games to a higher standard than we do our Sonics, Call of Duty's, or Mega Mans. I don’t want a new Mario game unless Nintendo can make me think "this is the best game, ever!" like they've done with every Mario until we got to the 2D, "Me too" feel of NSMB Wii or the 3D "Me too" feel of SMG2. Maybe it's bad for business that we don’t get a new 2D Mario, yet. As a fan of Nintendo, I'd rather wait until we can get another sublime Mario game instead of another "Me too!" Mario game.
And looking at the upcoming Mario game for the 3DS, I can see it's in 3D but maybe.....just maybe, it packs enough 2D gameplay and old school charm that it will appeal to both old school Mario fans as well as 2D and 3D camps. Maybe this is the Holy Grail of Mario that my great great grandfather used to tell me stories about (before we put him in that home).
I think the reason why NSMB Wii had much of a "me too" feel to it is because in essence it didn't offer much more than the DS game in terms of content. There were a couple of new power-ups and multiplayer, but that was it. I didn't mind though, because the game is totally awesome.
Anyway, what needs to be done in a new game is new world themes in addition to the old ones. NSMB Wii used the same ones as NSMB which is why it feels so "me too". If it had, for example, Giant World or something else, it would have instantly become more exciting.
Places you haven't really seen in Mushroom Kingdom before, some new power-ups even if they are restricted to only some selected levels (like Kuribo's Shoe in SMB3) or being rare in general (like the Tanooki and Hammer Brother Suits in SMB3), that would do the trick. SMB really doesn't need to hold on to its eight worlds formula, what if there were twelve? Or if the next game would be more of a New Super Mario World with one single gigantic world map featuring about 120 levels? The game could constantly awe you with new secrets that you didn't expect.
And just forget about this Holy Grail, it's not going to happen.