Skyward Sword is easily the most dense 3D Zelda game (ignoring the sky).  The ground areas are packed with enemies and things to find.  You would have to look to a 2D Zelda for a more dense, compact overworld.

Skyward Sword doesn't even have an overworld. They might as well have removed the sky part and opted for a level select screen. There are no villages, cities or even other races present in this world at all. Nothing in this game makes you feel like you're in a living breathing world. It's all a very linear and formulaic affair.

Well, this is for big fans (it is the best selling Zelda of all time) who want to be able to play the game again just more polished and cleaned up.  It's also for both fans who weren't over fond of all the waggle and people like me who were turned off by the waggle but want to give it another go.  I think the traditional controls with the un-mirrored world will be attractive to many.  No, it likely won't sell multiple millions.  But it will have it's fans and supporters.  I'm buying it.  Never did much with the Wii game and am interested in getting to play a cleaned up version with traditional controls and the gamepad's convenience features.

You're correct in that the Wii U's architecture should make the porting process quite straightforward.

However, I suspect the PC running some of these mods has a lot more available memory and fillrate than Wii U.

Mind you, I'm not saying that TPHD couldn't look better than it does on Wii U. It definitely could.

The overworld is everything not inside a dungeon.

All of civilization is in the sky in this game.  That was clearly a conscious decision.  Only the primitave Kikwis, Mogmas, and Parellas are on the ground.  Plus there is the ruins of the Sealed Grounds and the ancient civiliztion that was once in Lanayru.

The three main areas, Faron, Eldin, and Lanayru had more going on than any 3D overworld before them.  They are filled with enemies, puzzles, and collectables.  They felt much closer to the 2D overworlds that were so packed with content.   Ocarina of Time, Majora's Mask, Wind Waker, and Twilight Princess are all great games, but their overworlds are empty compared to Skyword Sword.

^ What he said.

One of Skyward Sword's best moves was to fill the areas land areas surrounding dungeons with puzzles, traps, etc, so that they serve as almost a blend of overworld and dungeon.

And there was plenty to make its world seem alive; there were four distinct non-human races, scattered collectables, birds and insects to catch...

Exactly. I have a hundred criticisms of SS, but that overworld, the ground specifically, is not one of them outside of the painful linearity. 

Yeah, I've been playing SS for the first time and I don't really have any problems with the overworld.  The ground is detailed and full of stuff.  And while the Sky isn't as detailed as the ground, I still get a kick out of the epicness of it.  I personally find it very cool.  It really just needed one or two big islands floating about with stuff to see or do and it would be almost perfect, IMO.  

On a side note, Skyward Sword holds up dang well.  

It's funny, when I first played Skyward Sword in 2011 I loved the game, but found its graphics ugly. I guess it was cos all through the Wii's life, "Zelda Wii" was talked about as the game that would truly push the system's limits graphically.

However, when I replayed it earlier this year, I actually appreciated its graphics more than I did 4 years ago, despite the passage of time and moving on to the Wii U in the interim.

I almost said that exact thing in my post, but deleted the line.