Zelda definitely could, but it doesnt really need to. Breath of the Wild might not have blood, gore, or vulgar language, but i would still call it a mature game. It requires a lot of thinking and quick reflexes if you want to avoid dying too much, which is what I would expect from a game targeted at an older audience.

Metroid however could really shine with a higher rating. It deals with bounty hunters, intergalactic parasites, the main villain in the game is a giant demon-like dragon. If they made an M-rated Metroid game and had the privilege of dialing up the gore and possibly even horror aspects of the game, I think people would be thrilled

M-rated Kid Icarus already exists - it is called God of War.

Metroid I want my aliens to be in agonising and visible pain when I shoot high energy plasma balls and rockets at them. Also imagine the psycho horror elements that a M Rated Metro I'd could provide. Also a mature audience should be able to solve harder puzzles. So a Metroid aimed at an 18+ audience would be nice.

Unnecessary. It wouldn't add anything to the games.

Well sometimes you have to say something useful  about Nintendo instead of trolling Nintendo. Otherwise bans will follow.

Eternal Darkness Returns!!!!

With the inevitable return and death of meta ridley. A bad ass line like are you going to return from death again to be killed by me all over again and again. Or do your disformed dragon balls or have you finally got enough. Before Samus is going to brutally kill Ridley by throwing it in the nearest sun. Or sticking a high intensitiy plasma bomb down her troath and melting the blowIing the beast up while still alive.

I just want to play good games.

I am aware.

I think Bayonetta fits this mold perfectly. :P