It's too bad Zelda doesn't perform like 2D Mario or even 3D Mario. I wonder what they are going to try to do to increase sales dramatically? Maybe a full blown top down Zelda?
Nintendo doesn't need to do anything since Zelda is still one of their top selling series. Look at the actual sales and you'll see the Zelda series has been pretty consistent in sales.
The Legend of Zelda - 6.510.000
Zelda II: Adventure of Link - 4.380.000
A Link to the Past - 4.610.000
Link's Awakening - 3.830.000
Ocarina of Time - 7.600.000
Link's Awakening DX - 2.220.000
Majora's Mask - 3.360.000
Oracle of Seasons and Ages - 3.960.000 (Nintendo combined the sales of both games so we don't now which sold more)
A Link to the Past / Four Swords - 1.890.000
Wind Waker - 3.070.000 (These were the sales Nintendo gave in March 2004, a year after the game was released so the games final sales are higher since Zelda games have legs but we don't know how much higher since Nintendo hasn't said anything since)
Four Swords Adventure N/A - (Never sold a million and Nintendo has never stated it's sales)
Minish Cap - N/A (It's been said it's done over one million but Nintendo has never given offical sales of any kind)
Twilight Princess - 7.140.000
Phantom Hourglass - 4.130.000
Spirit Tracks - 2.610.000
Ocarina of Time 3D - 2.490.000
Skyward Sword - 3.420.000
As you can see the Zelda series is still quite strong. Twilight Princess which was released at the start of the Wii's lifespan is the second best selling Zelda and Phantom Hourglass is the best selling handheld Zelda. Even Skyward Sword which has come out when the Wii sales are at their lowest, has still sold better then a lot of other games in the series and by the time it finally stops selling will end up surpassing several more as well.
The only thing Nintendo's going to do to make the next Zelda sell better is get it out earlier in the systems lifespan since as the sales show, the first Zelda released on each system sells better then the later games released on each system. No surprise since the first Zelda on each system usually comes out when that systems is having good sales while the later ones that sell worse come out when the sales are down.
This is no different then most other video game series as well. When a system is selling better the games on that system usually also sell better, while when sales of the system drop, so do the sales of the games. This is why Iwata himself told Aonuma after Skyward Sword that he want the next Zelda to come out in 3 years instead of 5 since Iwata knows the next Zelda will sell much better on the Wii U in 2014 then in 2016, since Wii U sales will more then likely be much higher in 2014 then in 2016.