It's just a few dungeons added and Majora's Mask. I'm talking about a games with all the great elements of 3D Zelda (cool weapons, cool locations, cool bosses, cool dungeons), but bigger, better, and badder. More difficulty levels (and actual difficulty levels, not "enemies do twice as much damage" crap), loads of side quests, a gigantic world to explore that is worth exploring, loads of enemy variety, more advanced enemies (in terms of intelligence, strategy, and the kind of fighters they are), and just pretty much a gigantic game that is hard to get bored in. That would be especially great if the sense of "adventure" that was there at many moments in the Wind Waker was present, and the sense of purpose was there, as in Majora's Mask.
Quite frankly, once you beat the main quest and fetch quests in Twilight Princess and Skyward Sword, the game offers nothing to you to do. That's unacceptable in this day and age from a franchise as great as Zelda.
Oh, and don't remind me about what Zelda could be if it went back to the exploration roots and played more like TES.
I don't think TES was that big of a deal, since all it was adding was more content, and the other half was Majora's Mask (which was a huge step forward for gaming IMO). I would have loved to see Nintendo add some of that stuff to Ocarina of Time 3DS, however. I felt Nintendo missed a huge opportunity with that. I hope they add tons of stuff to the Wind Waker HD.
Wind Waker has so much untapped potential, it's ridiculous. I mean for reals, imagine if that game's sea enemies and combat system was as diverse as it's on foot enemies, if that game had a harder difficulty level(s), if they added more locations and longer dungeons, and if there were several more cities and side quests.
I guess I'm both describing what the Wind Waker should have had (a bit), but moreso I'm describing what the Zelda games after the Wind Waker missed the opportunity to have to help compel those games to legendary heights.
Regardless of what the haters say, the Wind Waker did some very brilliant things, and was so, so, so close to bringing gaming to new heights.