Both overworlds definitely had their advantages and disadvantages, meaning there are certainly aspects of both I'd like to see in Zelda Wii U's overworld and others I wouldn't.
Twilight Princess had an immense overworld open to exploration; the problem was, there wasn't much to explore. There was the occasional Piece of Heart or cavern, but little else filled the gaps in the grandiose fields. The dull graphics of the game definitely didn't aid in this, but the fact remains the overworld was very plain - no pun intended. The ability to teleport that the player eventually gains is helpful, and so is Epona, but the overworld should be something greater than a large field that you want to get out of as soon as possible; it should be a land begging you to explore it.
On the other end of the spectrum, Skyward Sword was free of TP's overworld issues, but it still had its own. Instead of compromising, SS completely ridded of the overworld and gave us three hallways connected by the Sky. The Sky was compact with its hub of Skyloft and smaller islands floating around it with mini-games or treasure chests, but it certainly was not anything special or even worthwhile for more than the minute or two you're flying through it. The hallways limited exploration, but they also had their perks; the straightforward approach improved the transition from overworld to dungeon by adding puzzles and other neat gameplay elements, but the whole time it was hard to fight the feeling that the player was being guided by both hands. Like TP's overworld, SS's left much room for improvement.
So what can be taken from these two juxtaposing overworlds to fabricate a unique and superb overworld? The size of TP's overworld can definitely be utilized, as long as the space is filled with numerous caverns and sidequests that are fun and worthwhile to the player; I think an overworld between the sizes of TP's and OoT's could do this very well. I'd be pleased if Zelda Wii U expanded upon what TP attempted with its overworld instead of abolishing it like SS. That being said, the hallway system of SS can also be used; if the hallways are interconnected by the aforementioned overworld that is huge and filled to the brim with things to do, then the hallways can reach their full potential without the player feeling that the game's overworld is lacking. If these two elements can be blended together nicely, Zelda Wii U's overworld has quite a bit of potential. Another thing I'd like to see in the overworld is more cities than just one or two like most games have; if the game has three or four cities - for instance, a Castle Town and Kakariko Village accompanied by a port city and another village like Kakariko - not only will the overworld not feel as empty, but there will be more sources of sidequests and places for shops and perhaps even guilds like some players desire.
In conclusion, if Zelda Wii U is able to combine the best of the overworlds of Twilight Princess and Skyward Sword and add a few more features like caverns and cities, I will be very pleased with the final product.