Meaning of "D"

Ever since I started watching OP I always believed it meant that the D was a sideways smile like :D and that anyone with the D. Initaial was destined to die with a smile on their face.

Now much later in the story we are learning much more about what it actually means. We know those with a D. Are the enemies of gods and basically the celestial dragons.

Other than being a desendant of those inspired by Joy Boy, is there supposed to be any real significance or implied destiny?