Barring any crazy steampunk Retro-Zelda, the two clear options to me are Prime 4 or DKCR2. Honestly, I'd be happy with either (and it's possible that they could be making both). The Prime mechanics have some real possibilities with the Gamepad. Moreso than 2D Metroid, I'm sorry to say (but not TOO sorry, since I want to see that on 3DS with parallax scrolling). And I'm sure DKCR2 would be a super-solid game, especially if they took out the few annoying features of DKCR (you know which ones).
I'm sure someone in Retro, somewhere is working on some sort of game.
But even if they aren't and the above post is accurate, it may be of greater benefit to the WiiU in the long run. How often did we bemoan lazy third parties who clearly couldn't be bothered using the potential of the Wii? With Retro's guidance and encouragement, we may see more developers make a proper effort this time around.