I think it would be a great way to give us something else to strive for. The challenges don't have to be insanely difficult. They could just be simple, but time consuming like get a certain number of multi kills with Sunshot to unlock the Inferno ornament or something. Why stop at just ornaments? Legendary weapons could also get their own exotic shaders that could change up their appearance just as much as an ornament would, or just something a little more generic if that's too crazy.