We probably don't have to worry about New Super Mario Bros. series not stopping. Nintendo had a lot of time to pump out a sequel on the DS but they instead let the first game continue to sell at near MSRP. This has followed with the Wii games and will probably follow with the 3DS and Wii U games. So, hopefully Nintendo has NSMB out of their system for their current systems.
At most, there will probably DLC packs for New Super Mario Bros. U. I'll be shocked if there's a New Super Mario Bros. 3 on the 3DS or New Super Mario Bros. U 2 for the Wii U. That would be a bad sign of Nintendo's health.
Yeah, Nintendo has a clear track record of only cranking out one iteration its core franchises per console (same goes with stuff like Mario Kart), so I don't think we're expecting to see more of those games anytime soon. It was just the lethal combination of heaping them on so close together, and doing so little to change them up. Whenever they do get back to that series, they better take it somewhere new.
The problem I have is there still no Mario game where it incorpoartes all of the "moves" he's ever had. Not that I need the punching back from SM64(it was pretty dumb outside of the wallkick), but it would be nice to see that in a future game. Basically have more or less all of the SM64 moves plus the spin from SMS. Same thing in 2D, with NSMBU they brought back the SMW style Yoshi, but added flutter kick from SMW2 & SMS and the new fruit meter.
Basically what I want in a New/new Super Mario World game that takes all the good stuff from SMB3, the later Mario games, and put it into something like the Super Mario World engine and it is basically a Metroidvania but still has Mario like levels built into the world. Sort of like how Skyward Sword's world on the ground was already a sort of dunegon before you walked through some big door and had to fight a boss at the end. Anyway now that Miyamoto doesn't seem too hot on doing big fancy games anymore, someone else needs to take Mario and do something creative with it, and adapting the sort of openness of Galaxy and adapting it to a 2D game would be interesting, aside from just making it Mohawk & Headphone Jack with Mario.