If Fire Emblem 4 is the next game to get remade, I can see that easily get an M-rating. The first half of the game ends with nearly the whole main cast being burned alive, and the initial main villain is so obsessed with his banging his mother, he brainwashes his sister to bear his children after he sees how much she looks like her. And he succeeds.
Metroid is possible, though it seem unnecessary to me. Perhaps a future Xenoblade but same thought. I guess I'd just have to see how it's done before casting judgment, I just hope there's a solid reason for the M-rating and not just trying to get that rating for its own sake. There's a certain level of gore and nudity that I find is ironically less mature than your average E-rated game.