I have read or watched many theories, so I have sum up.
The Devil Fruit is the fruit that contains the will of people who have succeeded in evolution.
So, that will also be Haki, because Haki can negate the effect of Devil Fruit power if it is strong enough.
If you eat a Devil Fruit, it means you inherit that person's willpower.
Also, I really expect Zoro's Asura form to be an example of his desire for evolution.