I thought this box art was pretty believable. It is fan-made and in no way confirmed, but I kinda like it.
View attachment 16548 http://fc09.deviantart.net/fs71/f/2011/179/d/7/zelda_skyward_sword__box_art_by_shenani-d3k8zec.png
The reason why this is so believable is that it follows the golden box art trend that started with Ocarina of Time. It has the updated logo and dons an important feature of the game on it: Link on his bird. This can be compared to Link and the King of Red Lions on the Wind Waker box art or Link and Wolf Link on the Twilight Princess box art. I wouldn't be surprised if the final product looked similar to this...
This matches the trend of what we've seen with Zelda Box-arts. It's golden, and it's logo features an important story element, and below is another important point of the game. I wouldn't be suprised at all if the real thing looked similar to this...