Thriller Bark just so happens to take place immediately following the introduction of the concept of the ‘Four Emperors’ in chapter 432. We can, to an extent, gauge which ideas, characters, and concepts are interrelated based on the proximity with which they are introduced in the narrative. I can easily imagine Oda planning for Moria to play a critical role in Kaido’s defeat from the time he was first introduced.
As he collapses, Moria tells Luffy to go on and face the ‘real nightmares’ in the New World, again thematically connecting Nightmare Luffy to the ‘Dream of Wano’, the ghosts of the Akazaya Samurai, and the battle against Kaido.
Perhaps hinting toward the role this form will ultimately play, Nightmare Luffy wielded a large Katana. We’ve never seen Luffy wield a Katana before Thriller Bark. Again, in Act 1 of Wano, Luffy was running around with Nidai Kitetsu. Swords are regarded as vessels for the soul or will of those who carried them in Wano and thus wielding a sword to strike down Kaido will both echo Ryuma’s defeat of a Dragon and serve as a physical / tangible representation of Wano and Oden’s will.
It seems like the ideal solution. Toki’s prophecy foretells Nine Shadows striking down Kaido and Moria just so happens to possess a Devil Fruit which allows for the control and manipulation of shadows. There’s a clear difference between one person hitting with he force of nine and nine people hitting with the force of one.
Kaido’s greatest asset in battle appears to be his durability and so breaking through his defenses will be critical in achieving victory. Putting the force of all nine Red Scabbards (or all 9 of Luffy’s Worst Generation friends / allies) behind Luffy’s punches would certainly seem to be more effective than nine people independently beating up Kaido.
Nightmare Luffy allows for the defeat of Kaido without permanently powering up Luffy to the level of Emperor. In the eyes of the World Government, Luffy will be credited with Kaido’s defeat as the one to deliver the finishing blow, but we, as the readers, will know this is a form Luffy cannot access naturally and as such, still has room to grow through Awakening, Gear 5, etc. in the future when he faces Shanks and Blackbeard (and maybe Big Mom again).
*Theory by MeanMustacheMan / StrawHatJedi
