It'll be prettier and have some new visual effects. I wouldn't expect much change when it comes to core game itself. That's what a remaster is.

The differences in the music in WWHD were basically just instrument upgrades - some of the orchestral stuff sounds a bit more realistic.  One song that actually has a new part added is Tower of the Gods - there's a choir part that wasn't there in the original and it weirds me out every time i hear it.

I'm expecting changes more akin to the 3DS remakes than Wind Waker HD, which was closer to a remaster than a remake:

• Revamped textures, lighting, character models, and better animations for Link/Wolf Link. I'd hope for something similar to the 2011 tech demo.
• Hero Mode, available from the start.
• Option to choose between unmirrored, gamepad controls (with inventory management etc on the bottom screen) or mirrored, wiimote controls. 

I'd like to see some of the following, but don't really expect any of them to make the cut:

•  Orchestrated soundtrack. Twilight Princess would really benefit from that, the main theme in particular is stirring but the snythesised music has not aged well. There were some fairly ambitious sounding orchestral plans in place for TP, but Koji Kondo decided not to go through with them. I'm expecting we'll get an improved synthesised soundtrack, instead.
• Revamped tutorial. TP is very, very slow to get going. I don't mind it, but people were impatient with it 9 years ago and it's not as if the average gamer has become more patient.
• Boss rush. I enjoyed this in Ocarina 3D, disappointed we haven't seen it since.
• 'Tougher' enemies: Hero Mode will mean they do more damage, but faster enemies with more varied attacks would be great. TP has great looking enemies that often lack any real dynamism in defeating them. A shame when the combat has a variety of special moves to use.
• Revamp a couple of the boss fights: the third boss and the spider in the Temple of Time come to mind. I'd prefer a straight up fight with the spider, and a faster underwater fight, if possible. 
• More to do in Castle Town, even if it's just a few more minigames. 
• Overhaul the final stage of the final boss fight: it's incredibly atmospheric, with incredible music (even better if orchestrated), but it's not a patch on the final fight in Wind Waker. It needs to be faster, more brutal and much more intense.

1080p
60 fps

Bigger boobs for "sexy Midna".

That is all.

So more like this?

How about Link's Crossbow Training as an extra mode included?!?

... Anyone?

Not exactly what I had in mind but I like it!

I bought and liked TP - I think it has some really, really good qualities - but I stopped playing at a part where there was frustrating horseback riding combat. Something about it really annoyed me and I put the game down and never picked it up again. Always meant to, but it's been years now. So I may wait for an HD update.

I'd say I want improved graphics, tighter controls and better dark world zones. The second two items really annoyed me. I thought the graphics were beautiful as is, so only a mild update to HD is needed there.

I understand where you're coming from, and whilst I don't think that Nintendo would implement this, it's not impossible or as difficult as it might seem.

http://vignette3.wikia.nocookie.net/zelda/images/f/fd/Map_of_Hyrule_(Twilight_Princess).png/revision/latest?cb=20090330213854

If you start on the Great Bridge of Hylia, you can walk to the northern part of Hyrule Field, through the canyon (the one that leads to the Hidden Village), over the Bridge of Eldin, across the eastern part of Hyrule Field, and via the Kakariko Gorge and the southern part of Hyrule Field all the way back to the Great Bridge of Hylia without encountering any loading screens. The only concern would be whether Castle Town fits inside the space that is left in the middle. It's a pretty big map, so I imagine it would be possible.