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...

"A beautiful drawing in 480i will stay beautiful forever...
and an ugly drawing in 1080p will stay ugly forever..."







