I don't know why people are all of a sudden thinking there might be no "gimmick"
We have quotes from both Iwata and Kimishima stating that there will be "a brand new concept" and Iwata stated that they are reluctant to reveal early so that "competitors won't steal our ideas."
These aren't the kind of things CEOs just BS about, there is bound to be something unique about this system. Plus, Takeda (I think) was alluding that Nintendo always tries to think about new inputs or interfaces for their hardware, so I'm 100% sure there will be a new "gimmick" with this console. Hopefully a cheap-ish one.
If we look back to Nintendo's recent past control "gimmicks," the only one that I see which wasn't foreshadowed was the Wii remote. They played around with 2 screen gaming way back with the Gamecube and GBA, which eventually evolved into the DS line as well as the Wii U, so I think we can look to the recent past to see what kind of "gimmick" (or innovation, depending on how it pans out) they'll come up with this time.
I personally think it's eye tracking input onto a TV. The new 3DS has toyed around with eye tracking and they've filed a few patent applications like that, but we haven't explicitly seen any inkling or demonstration of direct input based on eye movement, which I think is very technically feasible and quite cheap, hardware wise.
But who the hell knows, they could pull out another Wii remote where the "gimmick" comes right out of left field!
PS: I personally still believe there will be a screen controller included in each SKU, though I think at this point a nice streamlined touch screen gamepad should be pretty cheap, and not the main "focus" of the "gimmick."