CoC is less based around actual power and more based around will power. His whole character is based on his will and how he wants to strengthen it. His goal is very ambiguous (becoming a 'Brave warrior of the sea') and it doesn't yet have a specific point of completion which I and many others think will be unlocking CoC and proving that he's strong-willed.
Also, there are hints all along the story. I bet you know of the thing about Usopp's lies coming true. Well, guess what, he lied about having CoC in Dressrosa. I personally think that it'll be a great end to his character arc. Also, don't you underestimate Usopp.