Nintendo has said they will never do DLC that costs money. This doesn't rule out free DLC.
That hasn't stopped them from releasing 10 separate DLC stages for New Super Mario Bros 2, each costing $2.50
It hasn't stopped them from releasing dozens of DLC for Fire Emblem Awakening, at $6.50 each.
Have you checked your Street Pass lately? 4 DLC games for 5 dollars each.
Final Fantasy Theatrythm sells each individual songs as DLC.
Gamefreak is not immune. Pokemon Mystery Dungeon had paid DLC. You could add extra areas and dungeons for 2 dollars each.
These are all 3DS games. There are many many more, be it first party, 3DS exclusives, or a simple port.
I wouldn't be surprised if Pokemon X/Y took a DLC approach. Maybe they won't even release X2/Y2 or Z or whatever they'll call it. It'll just be a 35 dollar DLC instead.