As far as an M rated Zelda game goes, we can never be sure. Although Nintendo would be taking a risk, it isn't unheard of to have M-rated games from Nintendo. But, it would take a lot of change to the series for that to happen.
From the ESRB website: Titles in this category may contain intense violence, blood and gore, sexual content and/or strong language.
Okay, so Zelda has a lot of violence. Fantasy violence. Meaning that, so long as we don't have a lot of blood and gore, nobody will notice that we've just killed thousands of villains. Speaking of blood and gore, I don't see the Zelda games making it into an M-rating in this category. How many Zelda games do we see where, after hitting an enemy they bleed, and once you defeat it, the body remains? Exactly. This would change the gameplay drastically. Sexual content... well, I've seen a lot of sexual content in T-rated games, so to make it an M-rating in terms of sexual content would be very difficult. And strong language, well I don't think we'll see any moblins running around throwing out the 'f'-work much.
However, this doesn't mean that Zelda will never make an M-rated game. Perhaps Nintendo will think, since their original fan base (from the NES days) are adults now, maybe they should cater a game specifically to them. However, this would be to exclude their newer fans, which I'm sure wouldn't appeal to Nintendo. Or they'll decide, hey, why not make a game for all these older gamers, who think Nintendo is a 'kiddy' thing? But, of course, Nintendo isn't one to make an M-rated game just to get more players. They certainly aren't lacking in Zelda fans, that's for sure.
But, once again, who knows where Nintendo will take us next? That's just who they are, always surprising fans. If they did create an M-rated Zelda game, yeah, I would buy it. But, with slight skepticism, as I'm sure many would, because they would have to basically change so much about the game to make it so highly rated.