The next Zelda being full NX exclusive would guarantee sales, but piss off the fanbase. A better choice, and most likely, will be a Zelda U port.
Also there must be a Mario 64 caliber game. The WiiU did not have anything like that, so fans will flock to it. A new Mario 3D world would be nice as well as that caters to a different style, but an actual adventure Mario is a must.
A proper metroid, current gen Donkey Kong, Earthbound, and a follow up for Star Fox announced for later in the pipeline. And last but CERTAINLY not least, a true console version of Pokemon with full online support (main game to be single player though).
If they do all of this, they could make a monumental comeback. If they do half of this at launch, they will at least be relevant to most of the console market again.