Nintendo it seems has long been against DLC in their own games so as to not “cheat” the player out of content. However, this may no longer be true as far as the Wii U is concerned.
Nintendo’s Official Magazine reports that the development team behind Super Mario Bros. U is already working on extra stages and gameplay elements to be released in the distant future.
I totally get they would not want to release DLC for Animal Crossing because it could turn it into a Free to Play type game very quickly. With Mario I don’t see why not, I mean sure it is Mario and people will be up in arms about it at first. However, why would you not want extra stages for a game? You never know, Nintendo might even offer the extra DLC for free.