I'd like to see a 3d secret of mana. I think that would fit in the Nintendo universe very well. Just borrow the time, the co-op, the cutesy feel, the weapon system and you've got gold. I'm not sure how well it would sell, but it'd be a ton of fun to play. As an added bonus, allow the game to switch between 2d and 3d at will to really give it a kick in the nostalgia pants.