I don't really need an "M rated" game. You don't need blood or nudity or swearing to entertain me, but you do need to make the content deep and complex. Typically T ratings or lower come with games which don't care.
Zelda already has done a clean "Almost Should be M-rated" game; Majora's Mask. It's a cult favorite in the series and I totally think that clean M ratings would be a fantastic direction for Zelda to go in. The same could arguably be said about Fusion with Metroid, but not to the same extent. Fire Emblem would actually need the rating and to not be clean to make it's point.
Picking one to go true M-rating? Metroid. We might get to see Samus in an imaginary suit. I know I said I didn't need that, but I lied.
Hey, it's a long shot, but I can dream, right?