I think we can all agree that the NSMB games didn't need to be stretched out to four games and a full length DLC. And 3D Land and 3D World were essentially also NSMB games whose difference was that it occupied a 3D space and cost us the high profile 3D Mario games that this generation desperately needed. The point is that, up until NSMB, no Mario game was the same experience as another. Each game (SMB1, SMB2, SMB3, SMW, SM64, SMS, SMG) all built upon series staples while offering a fresh new experience. Gameplay changed from game to game.
Mario 1 introduced the standard gameplay.
Mario 2 completely threw it out the window.
Mario 3 brought a lot of variety to the standard gamepla introduced in Mario 1.
Mario World put the focal point on having a large overworld with multiple routes to take.
Mario 64 revolutionized the gameplay by changing the focal point to collecting and exploration in larger 3D levels.
Mario Sunshine built on Mario 64 but introduced the FLUDD mechanics and a more in-depth story sequence.
Mario Galaxy took the collecting aspect of 64 and Sunshine but changed up the level design to be more direct but with a huge focus on gravity manipulation and added a bit of puzzle solving into the mix.
All NSMB (And 3DL, 3DW) did was take the series staples c. Mario 3 and just generated new levels for them. The only thing Nintendo did was to differentiate them was introduce two new powerups each time. (NSMB = Mega Mushroom + Mini Mushroom, NSMBW = Propeller Shroom + Penguin Suit, NSMB2 = Racoon Leaf + Gold Flower, NSMBU = Super Acorn + Baby Yoshis, SM3DL = Tanooki Leaf, Boomerang Flower, SM3DW = Super Bell + Double Cherry).
Aside from the two new powerups per game, they were all disappointingly similar with 8 (give or take) Worlds with predictable themes. You beat the boss at the end of each world by jumping on their head three times, rinse and repeat until the credits roll. That's why I've found Captain Toad to be the best Mario game this generation. Because it's different.
So if Nintendo needs some time to take a break with Mario, maybe if the series comes back in 2016 at the earliest (making it a three year gap between Mario releases - the largest gap in Mario releases since Sunshine and NSMB.) Nintendo will finally have some new ground to tread.
I'm still maintaining that an open world Mario would work if done correctly.
Edit: Oh, and IIRC, Satoru Iwata has stated in the past that the NSMB series is only going to get one title per console- which implies that if a new Mario game comes before the next console- there's a good chance it'll be different. inb4 3D World 2 though.