You probably remember some videos that surfaced for a Zelda game back when the GameCube was brand new. Ultimately I think the developers kind of took on more than they could reasonably handle, and the project was cancelled. Much of it probably went into what became Twilight Princess. Which was quite a bit darker and moody compared to say Wind Waker. Although the bit at the end of Wind Waker, where Link runs Gannondorf through the head with the Master Sword wasn't exactly a light hearted romp.
I'm right there with you wanting something of a more mature Zelda game. At least maybe PG-13 level if not R. They don't need to go over the top with gore everywhere, but they could get rid of the annoying NPCs, and the dumb fetch quests like dominated Twilight Princess with the killing of the shadow beats that stole the light from the regional guardians. Unfortunately, it's unlikely to ever happen with Nintendo. Their bread and butter is the pre-teen segment, and for the longest time, Nintendo refused to allow any M rated games on their consoles. I highly doubt they'd take one of their biggest franchises and try and tinker with the formula much, even if that formula has gotten a bit stale over the years.
I seem to remember seeing a video showing off some realistic, when compared to the past games, footage of a new Zelda game to be aimed at the older audience. I have yet to hear anything about it since then though, and its been quite a while. Was this all in my imagination, or is this really in the works?