Haki Evolution

Haki Evolution (覇気進化, Haki Shinka; Japanese for "Evolution of Ambition") is a phenomenon coined by Kurama in which the of an individual who has eaten a  undergoes a significant metamorphosis as a result of sufficient familiarity with the fruit's nature. It is literally an evolution of a living being's Haki, acquiring traits that are associated with the Devil Fruit they have consumed.

Overview
Haki is a base power found in all living beings, for it is in essence the very manifestation of an individual's own life force in a form suiting their needs. It is the manifested form of their own willpower, and as such, it embodies the nature of the willpower of that living being. For a human, it is the "will of a human", and for animal-like species like the, it can be said their Haki is a reflection of their beastly nature. While for most living beings, regardless of their breed, Haki is often the same and is expressed in the same way, those who consume Devil Fruits and acquire some degree of mastery over them soon realize that in addition to being changed themselves, even their Haki has undergone a change as well.

When a person eats a Devil Fruit, everything about them changes. Their body transforms into an element, substance, or creature of a particular type. Eventually, however, they come to realize even more things about them have undergone a transformation. Haki itself is the expression of life energy, and with the Devil Fruit user no longer being a "normal person" but now a reflection of the fruit they have devoured, it can therefore be said that their life energy is also a reflection of this newfound nature. Someone who has consumed a tiger Zoan fruit may eventually be able to utilize Haki that reflects the "will of a tiger", symbolizing aggression and a predator instinct which could reflect in an enhanced Kenbunshoku Haki and perhaps a far more potent Busoshoku Haki.

Kurama theorized at one time that Haki Evolution may be the reason for how Devil Fruits achieve "Awakening".
 * Zoans acquire a will reminiscent of an animal, embodying "instinct" which causes them to acquire more monstrous properties that can cause inexperienced users to lose control and go berserk.
 * Logias acquire a will that reflects their elemental nature. A fire fruit, for instance, embodies the "will of fire", spreading out and burning everything that it can, producing flames that will continue to burn until whatever it clings to is completely turned to ashes.
 * Paramecia acquire a will reflecting the substance they have become. By projecting this will outwards through their Busoshoku, they essentially imbue their Haki into the surroundings and cause everything within their sphere of influence to transform into the same substance.

Behind the Scenes
Just a theory for now, the concept is still under development by the author.