The main thing with Zelda, I'm hoping for anyway, is utilising all those extra hardware resources to make more detailed and intricate game worlds. Miyamoto's original vision for Ocarina of Time could actually come to fruition.
On a completely random note, and this is going to come across dickish and ignorant, but I'm a wee bit surprised the Wii U is using 25GB disks. It makes me happy, but given the 360's success I totally wouldn't have been surprised to see them cheap out and stick with DVDs.
Well, when you say more detailed and intricate game worlds, are you also referencing its physical size? I personally dream of an open world Zelda game with the detail and scope similar to World of Warcraft (Oh so you want to get to the desert...yeah you don't just hope over to the next leading zone, you're gonna have to riiiiiiiide). Or do you just mean detail as in more vegetation, more intricate polygon models etc.