Voice acting. Maybe it's heresy, but if there's meant to be a big change in next Zelda, why not have it be this one? Nintendo is often criticised for not having enough story and atmosphere in their franchises. Zelda and Metroid have always kinda countered this argument, but an even more story-heavy Zelda (with actually SURPRISING plot twists) would be the best thing Ninty could do IMO. Voice acting is an essential part of this.
Atmosphere is also important - Twilight Princess already was a big step into a darker direction, and I'd love to see this trend continue in next Zelda.
Nostalgia - seeing the Temple of Time unveiled in TP sent shivers down my spine, remembering those blessed days of Ocarina of Time. I'd like to see more of these nostalgic references. Zelda has more history than virtually any other franchise, and should exploit this some more.
And finally - personally I wouldn't mind a return to the post-Hyrule Great Sea that we saw in the Wind Waker, but without the cartoon art style. I loved exploring the islands and the vast ocean, and it would be even more complete in a darker, moodier setting. If we could have a huge OoT/TP-style Hyrule Field AND an ocean, the dream would be complete.
I drink your milkshake.
