CoC would make much more sense to be something you can achieve by changing your mindset throughout your life, and not something you need to be borned with it.
That makes it be all about chosen ones, which is bad.
And then CoC coating is simply a better CoA LoL
CoA is good concept, the way Oda treats it that doesn't makes sense, "visible vs invisible haki" "Was he using haki here?" "He was using haki, look at his arm, its black" "Why aren't they using haki?"
Its messy as hell
CoO only exists when plot demands it in the most cases