In the latter parts of the series, the sun has become a major motif. You have Nika(who in all likelihood is joyboy) being the sun god, Luffy bringing the dawn(which has been mentioned throughout zou, WCI, and wano), and blackbeard, who represents the darkness in opposition to the sun.
Because of this, I believe Luffy's final and greatest power up will be sun related. That at its highest possible level, the spirit of a CoC user becomes so strong that it essentially radiates with light and heat. Nika, having mastered this level of haki, was known as the sun god. In order to defeat blackbeard and the powers of the yami yami no mi, Luffy will also attain this power.
In fact I wouldn't even be surprised if Luffy already has it. Red Roc and Red Hawk both use fire through unexplainable means. I think the highest level of CoC would essentially be a completion of this ability.
Furthermore, Luffy gaining this power would cause him to parallel in many ways to Ace, who had the mera mera no mi, a similar fire related power. It would signify Luffy inheriting Ace's will and would especially be poignant in the battles with akainu and blackbeard.
Because of this, I believe Luffy's final and greatest power up will be sun related. That at its highest possible level, the spirit of a CoC user becomes so strong that it essentially radiates with light and heat. Nika, having mastered this level of haki, was known as the sun god. In order to defeat blackbeard and the powers of the yami yami no mi, Luffy will also attain this power.
In fact I wouldn't even be surprised if Luffy already has it. Red Roc and Red Hawk both use fire through unexplainable means. I think the highest level of CoC would essentially be a completion of this ability.
Furthermore, Luffy gaining this power would cause him to parallel in many ways to Ace, who had the mera mera no mi, a similar fire related power. It would signify Luffy inheriting Ace's will and would especially be poignant in the battles with akainu and blackbeard.