I'm still curious about the new Amiibo functionality. They might ultimately be more useful than the treasure chests of promotional clothing. When I first read these items, I thought I would just buy the story part. Hard mode is only interesting if it is significantly different, as opposed to enemies taking more damage and that sort of thing. If I want that, I'll just play the whole game naked.
At $20, the extra story content needs to be very substantial. Since most of us will have beaten the main game and most of the side quests, having the end reward be a special weapon or armor would be meaningless. It just needs to be fun and compelling for a long time. If they do that, and it doesn't feel like they just left out content that should have been there to begin with, then I'll be happy with it.