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Zoan Awakening is the User reaching a Level where his Body & Mind can fully embrace their Zoan Being
It's like imagine when someone eats a Zoan, they can Transform into that Creature but their Body & Mind don't Harness it's Full Potential
Just like Kaidou Explained
In the case of Normal Zoans, such Concept doesn't seem very difficult, it only requires Training & Experience
But in the case of Mythical Zoans who are basically Fantasy Creatures, it gets complicated, each One will have different rules on how to achieve the Right State of Mind & Body to Awaken them
Since Luffy required to Die to "Awaken" Nika DF
Means that only way for Body & Mind to fully grasp Nika's Full Potential, it needs to experience "Death"
Conclusion? Sun God exists in Afterlife, not in this World
This was even Hinted in Jaya Flashback when Calgara's Daughter said that she doesn't mind Dying cuz she will see Nika
Elbaf also Hinted more when they said that they celebrate the Rebirth of the Sun
But if you check other Mythical Zoans, you notice a Pattern
Azure Dragon is known for it's Tough Skin & Kaidou was famous for being Near Unkillable
Orochi's DF also gave him some sort of Immunity to Death. Same for Marco
However not just these, Buddha is related to Cycle of Rebirth, Devon's & Onimaru's & Yamato's DFs represent Spirits while Stronger is the Horse of Doc Q who is known as Death God
So one way or another, all Mythical Zoans are related to Death & Afterlife
So it's heavily suggested that only way to "Awaken" a Mythical Zoan is to Die
According to Brook, going to Afterlife & returning made his Soul Stronger & it was able to make his Skeleton Body do everything a Normal Living One would do, so this is clear indication that Afterlife does Buff Souls if they manage to return & apparently Mythical Zoans can do that.
So the only way for a Body & Mind to handle Awakening of a Mythical Zoan, it's Soul must be Buffed through going to Afterlife & returning
So to Answer your Question:
Luffy isn't Immortal but he did Die & that's what Awakened his DF
Kaidou wasn't Awakened not cuz he wasn't Strong enough or lacked Experienced but he needed to Die which he failed to do
However it seems that Kaidou, Orochi & Stronger finally Died recently, meaning that there is a High Chance that they will return Awakened!
We also conclude that such Zoans are called Mythical cuz they don't exist in Real World but actually in World of The Dead, they are Creatures who only survive in the Environment that is Afterlife. This should also be related to Zoro's Encounter with the Shinigami
The Fact that Zoro was able to survive that Encounter means that he follows the same Rules as Mythical Zoans, but he isn't a DF User, he is instead the same Type of Beings that those Zoans allow you to transform to (Such as Nika, Phoenix, Azure Dragon, Buddha ... etc)
And him being Asura is more than proof
Oda is basically indirectly telling us that Zoro also belongs in "Afterlife", this isn't his Home
And it was hinted since Volume 01 when we learned that Zoro is really bad with Directions
Do you know what else can't tell Directions? A Normal Compass in Grand Line
We were told that a Compass works in Four Blues but doesn't in Grand Line & needs to be switched for a different one
And then when they entered New World, they had to switch to an even bigger Compass (A Trion One)
So if Zoro's Sense of Directions doesn't work anywhere, then that means he doesn't belong in this World at all
This is another Major Hint that Zoro's Sense of Directions works in another Place & that is "Afterlife" (He is basically the Crew's Top Compass, a Nine Directional One)
I promise you that Next Arc is gonna be Mary Geoise & it's gonna focus on Navigation, Brook, Mythical Zoans, Double-Layered Eyes, National Treasure of Mary Geoise, Blackbeard, Death & Rebirth, Bink's Sake & it will all revolve around "Afterlife"