graphic violence and nudity are not actually what make the game for adults de facto. if those elements have no substance around them, it only appeals to teenage audience.
you need drama and/or philosophy to cater for adults, and you can mix violence and nudity there too, more or less, depending on what you are going for. you can make a lot motionally-heavier content without showing as much graphically, than you could by going purely graphical.
the thing is, that only the graphical content usually get's games banned.