"What's next for 2D Mario Platformers?" Well, probably nothing more than what NSMB and NSMBW are: Fun new games that are nevertheless basically interactive nostalgia trips. Why do you think they gave the first one the half-joke name "NEW Super Mario Bros."? I don't think it's impossible, but I think it would be hard for a new 2D Mario game to really innovate, for the same reason it's virtually impossible for a 40-year-old guy to sound sincere when he's talking to kids and says stuff like "that's tight"--3D is/was the natural progression past 2D, and a dimensional step back means that, if NSMB/W were marketed as to SMG what SMS was to SM64 (the next "real" Mario game), I think people would say "you're kidding, right?" The whole point of this two-game (as of now) series is a breath of old-school fresh air.
So is all that a problem? Heck, no--NSMB and NSMBW were both fun, it's just that I wouldn't consider either to stand on the same historical step as SMB3 or SMW, for the same reason a "Collector's Edition" rerelease of an old comic book isn't worth as much. But hey, maybe my mind will change over time.