For some reason this 'M-rated Zelda' concept gives me a vision of 300 Hylians, and Ganon totally naked aside of some golden jewelry covering his junk.
In all seriousness though, some of the most powerful motivating criteria in fantasy come from visuals that only T or M-rated games can provide. I think a touch of blood here and there, or perhaps a deeper development of Link as a character (or Ganon maybe?) insofar as WHY Link is the unflinching paragon of justice that he always seems to be. Even if the answer is 'Oh he has the triforce of courage, remember?' Then WHY does he have the triforce of courage?
The other thing is, Ganon is sorta getting old. That kid from Majora's Mask was a really cool villain. He had sensible motivations. He went from being a cold, lonely skull kid to a demigod, which makes for a wonderful antihero.