Nintendo mentioned in their last Nintendo direct, exclusively for Hyrule warriors, that the game is going to have one of these 3 pre-order bonuses: An Ocarina of time costume pack, a Twilight Princesses costume pack and Skyward Sword costume pack all for Link and Zelda.
I was just checking some online websites and found 2 websites so far that have posted their pre-order bonuses.
First Gamestop: http://www.gamestop.com/wii-u/games/hyrule-warriors/115561#bonusFeatures
They have the Ocarina of time costume pack.
Second Amazon: http://www.amazon.com/Hyrule-Warriors-Wii-U/dp/B00KWEHBAG/ref=sr_1_1?s=videogames&ie=UTF8&qid=1407533895&sr=1-1&keywords=hyrule+warriors
They have the Twilight Princess costume pack.
Unfortunately, those are the only ones so far that I have found. Iam looking forward to pre-ordering from a store that has the Skyward Sword pack, but I dont think it matters since I will be getting them all. Big fan :D . If you have pre-ordered the game already, you might want to cancel that and get it from another pre-order, unless you are getting all the costume packs hopefully when they get released on the eshop.
IMSM
