It seems any chance that Retro could be working on a Zelda game has been put to rest for now. Shigeru Miyamoto has ruled out why Retro isn't able to work on Zelda, and he naturally wouldn’t say what the team is making currently.
Miyamoto recently said:
“Retro is very busy right now, but I’m afraid I can’t say exactly what it is that they’re very busy [working] on.”
Miyamoto is still open to the idea of retro working on Zelda but besides being busy the other issue is that the Zelda team in Japan would need to be in constant contact with Retro (which is located on Austin Texas). Miyamoto feel he will need to move to Texas since he would be asked to be more involved with the game
Still, Miyamoto said that Retro is “too busy for that sort of thing right now.”
“In terms of them working on a Zelda, it’s not out of the question, certainly, for them to work on an entire Zelda game amongst themselves. Traditionally I think that the Zelda team has always had a close contact with anyone who’s working on a Zelda game. If you were going to have that happen in the US at Retro, that would be kind of difficult for them to be able to coordinate. Certainly they’re too busy for that sort of thing right now. It would probably require me to be involved to a great extent as well, so I would have to get over quite a bit too. I’d probably have to live in Texas… [laughs]“