Most of us already heard about this topic and a lot of theory has been made regarding it.
Some people tend to interpret this literally.
While it could be true, I tried to offer another interpretation of it:
"Sanji will fuse with Mythical Fiery Bird" could just a symbolic or at very least has some symbolic meaning in it.
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It could means that Sanji will assume control over Garuda's (mythical fiery bird) Kingdom (The Germa Kingdom) and all of it's technology.
He will has the current Garuda's (Judge's) will in him, maybe in the form of certain enhanced scientific modification of human in his body.
This is another interpretation of what "Sanji will fuse with Mythical Fiery Birds" would be.
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Since we assume that All Blue will be created when red line destroyed, then it will be a very strategic area for the traffic around the world.
Sanji, who dream about All Blue since his childhood, might also be the protector of this very strategic area that was placed at the center of the world.
Hence, the sentence "Sanji will maintain the balance of the world" is fit to that role.
He may use some of Germa's technology to protect what is meaningful for him, The All Blue.
Also, since Garuda is also known as the protector, this title may fit to Sanji's role in the future.
He may be later known as "The Garuda", thus strengthening the association of Sanji with Mythical Fiery Bird.
Some people tend to interpret this literally.
While it could be true, I tried to offer another interpretation of it:
"Sanji will fuse with Mythical Fiery Bird" could just a symbolic or at very least has some symbolic meaning in it.
_____________________
It could means that Sanji will assume control over Garuda's (mythical fiery bird) Kingdom (The Germa Kingdom) and all of it's technology.
He will has the current Garuda's (Judge's) will in him, maybe in the form of certain enhanced scientific modification of human in his body.
This is another interpretation of what "Sanji will fuse with Mythical Fiery Birds" would be.
______________________
Since we assume that All Blue will be created when red line destroyed, then it will be a very strategic area for the traffic around the world.
Sanji, who dream about All Blue since his childhood, might also be the protector of this very strategic area that was placed at the center of the world.
Hence, the sentence "Sanji will maintain the balance of the world" is fit to that role.
He may use some of Germa's technology to protect what is meaningful for him, The All Blue.
Also, since Garuda is also known as the protector, this title may fit to Sanji's role in the future.
He may be later known as "The Garuda", thus strengthening the association of Sanji with Mythical Fiery Bird.
