Now, I'm a purist, and I love my Zelda games left generally the way they were. I'm actually hoping it's nothing beyond Master Quest and new interface options; the fact that it has improved graphics, 3D, and sound was plenty for me, anyway. I just don't want anything throwing off the balance of the game, and I suspect they'll be wise enough not to put the extra content in the main game, as that would disrupt the story and the flow that has been treasured for all these years.
I've been a bit out of the loop, but Master Quest seems a pretty substantial addition. I doubt it would be a new dungeon (let alone several), because that's something that Nintendo just doesn't do very often. They did create a linked functionality that gave access to a new "dungeon" in the GBA port of ALttP, but again, that was a dungeon that was
very out of the way from the main quest, and it didn't really upset the flow of the game. If they do add a new dungeon, I suspect it would be something very cryptic and out of the way, much like ALttP.
A linked functionality with Skyward Sword isn't altogether unlikely, and that may not be far off. However, I don't suspect it will be anything beyond a few Easter Eggs, or perhaps a few optional character model changes, not dissimilar to what was done in Metroid Fusion and Metroid Prime.
As for changing dungeons, I think that would be somewhat pointless, especially if Master Quest is included. If they want to stay true to the original, I imagine Master Quest will be the extent of extra dungeons, because that is already a shift in the dungeons. Why include a third, different alteration of the dungeons? On that matter, I think the notion of adding a "Ura Zelda" is a moot point -- as JamesBond007 said, Master Quest
is Ura Zelda. The only known extra content that didn't make it into the game was the "Triforce Quest," and I'm almost positive that will remain on the cutting room floor. This is because it just simply doesn't fit with the Zelda world that's been built up around OoT ever since, and a Triforce Quest would be an enormous shift in story and would render the plot of the game egregiously sloppy, as the whole Triforce element of the story is completely flipped around. I just don't see it as likely at all.
The most I think we'll see is an extra side quest, and that is fairly feasible; if I had to guess, I would think that it may be an extension of the Happy Mask side quest, with potentially more masks.
I remain stalwart in my defense of the game, and I don't think they will change much, nor is there necessity to change much. Would you
not buy the game, after already having played the original, even if there wasn't any new content? I think new controls, interface, sound quality and graphics are major contributions, and you'd be lying to yourself if you would say that wasn't enough to make it distinguished enough from the original to warrant a purchase. Should we expect the game to be better? No, I think that would be something only to set yourself up for disappointment. I expect it not to be better, but merely a different spin on something that I remember and held dearly, to bring me a great sense of nostalgia;from each moment in the game, I'll be waiting eagerly to see what subtle changes have been made in the next moment. Hopefully, enough that I'll remember it in the same light.