1. What is Haki and How it is related to Devil Fruits?
Let’s start. So we all know that Ace has the Fire Logia Devil Fruit and he died in front of Luffy.
Later on, in One Piece 3D2Y anime special with Byrnndi World as the main antagonist we first saw Luffy using Fire in his attacks, which also surprised him and guess what he says “Ace’s fire” at very moment he used it for the first time unknowingly.
And we all excepted it as we’ve seen Sanji using fire in his attacks.
My point here is, could it be that when Ace died, he willfully gave some part of his devil fruit to Luffy being a caring, loving big brother and to protect him in his future fights?
But, it raises a question: What is the connection between Devil Fruit and Haki and is it possible for a Devil Fruit user to transfer his Devil Fruit power to some extent, not fully, to another person WILLFULLY?
I think it’s possible only for a person with very strong WILL/HAKI and we all know Ace was the son of the Pirate King Gol D. Roger. So, it can be possible that he possessed that level of Haki and inherited his dad’s will even though he didn’t knew it as he hated his father and didn’t get to realise it, nor we saw him using any haki consciously. Have we?
So, my speculation is Devil Fruit powers can be partially transferred to a person by Devil Fruit user with strong Haki.
2. What might be the possible level of Shanks’ Haki?
So, Shanks is introduced to us as a only normal human being (my speculation only) and yet a Yonko/Emperor of the Sea.
Whitebeard even mentioned it when Shanks went onboard his ship and knocked off his mates and the ship itself began to take physical damage from Shanks’s mere presence.
It is really out of question in here that Shanks’ Haki is super-duper strong. Maybe the strongest Haki in the series.
How he is so powerful? I guess that Gol D. Roger inherited his will to Shanks who was able to understand it and grew it and just like someone takes care of a pet, I mean it literally grew in power and its effect.
I think Shanks’ Haki is so powerful that he can not only control the will of weak minded people but also the Elements of a Logia users or Paramecia users.
I think Shanks can also dominate a MONSTER, I mean someone with Zoan Devil Fruit.
And that’s what makes him Greatest among Greatest like Whitebeard, Big Mom and Kaido.
Now, why still Whitebeard was the greatest and strongest pirate alive? I think because of his Devil Fruit and his physical strength.
Whitebeard’s Devil Fruit was Tremor-Tremor Fruit(Gura Gura no Mi) and it’s not a natural element. So, Shanks can’t just dominate him in power and he also had a level of Conqueror’s Haki.
And same goes for Big Mom.
But for Kaido it’s different and that’s how Shanks might have dominated Kaido’s will to go to Marineford as he is a Zoan Devil Fruit.
3. What can be the real meaning of having greatest Haki amongst greatest?
I hope you people remember what Don Chinjao said to Luffy back in Dressrosa Arc when both are fighting and Don Chinjao finds out that Luffy uses Conqueror’s Haki too.
Don Chinjao said to Luffy exactly this: “Can you become Greatest amongst Greatest Haki users in the New World?”
What does it means? For what I am observing from all the chapters and the story it can mean that a person having the greatest HAKI/WILLPOWER can dominate the person with Devil Fruit and/or superhuman strength.
Why I said so? It’s because Gol D. Roger was the greatest pirate and he certainly knew something about WILL OF D. and his power as well and he might be the most probable person to tell Rayleigh and his other crew members about Haki and how to grow it.
What do you think? Who has told Rayleigh and Shanks about Haki and how to master it and all kinds of it?
Do you think, Ace’s will is with Luffy in form of the fire he uses post-timeskip within his attacks?
*Theory by mmsking