There are many things i wish to find in the next Zelda: immersive controls, different difficulty settings and harder fights, the usual clever puzzles and amazing dungeons, and a great story...
But "mature theme"? What does it mean? Is it just the look? I know many didn't like Wind Waker, but i think it's beautiful and gorgeous, a real masterpiece... not really "mature", maybe, but i think's it's a shame some people thought the game was "kiddy" because you played as a very young Link...
That said, i've liked Twilight Princess as well, for the story, for Midna, and for some of the most atmospheric landscapes ever found in any Zelda... but i don't think it was more "mature" than any of the others in the serie, it just had a more "serious" and "classic" look, and a slightly darker tone, that's all...
Majora's Mask was maybe the most "dark", but even then, it's still a very light fantasy when you compare it to most sci-fi books with "real" mature themes... the Zelda's have never been "that" deep, and anyway, it's not the reason why i like the serie so much: it's all about the gameplay, the exploration, the immersion, the epic adventure, and the fun...
So, i wish the next Zelda will be colourful and cel shaded like Wind Waker, in an even more impressive way... young Link, old Link, i don't care... with a "mature theme" and a "mature story", why not, of course... but no need to be dark and brown to achieve this... and i'd still like to laugh a little bit: Wind Waker didn't take itself too seriously, and was really funny, and really fun... and that's one of the many reasons why i've enjoyed it so much...