I believe that the New Onigashima Project is actually Kaido’s plan to activate the Ancient Weapon Uranus. The Skull on Onigashima is actually a part of Uranus.
I suspect that Kaido’s true motive of moving Onigashima to the Flower Capital is to reunite the Skull with its Skeleton, which is been sealed underneath the Flower Capital. Once reunited with the Skeleton, these two fragments of Uranus will combine to awaken the Gashadokuro, which is a demonic spirit in Japanese mythology resembling a giant skeleton.
Kaido has repeatedly insisted that Wano is important to him, and considering his current intent of destroying most of Wano’s people and culture, we can assume that they are not the reason why. I suspect that Kaido chose Wano because it harbors Uranus, just like how Crocodile chose Alabasta for its containment of Pluton.
Kaido made it clear that he intends to use the Ancient Weapons in the imminent global war, which heavily implies that he is aware of the whereabouts of at least one weapon.
He has been using Onigashima as an effective base for years, so why does he suddenly feel the need to move it to the mainland? If you think about it, it really doesn’t make sense. I think that he has been saving this event for two decades, because once the Skeleton reawakens, it is nearly impossible to seal away.
In fact, this might be a very significant part of Wano’s history and why it looks the way it does. Awakening Uranus is a big deal, but Kaido already plans to instigate the biggest war in history, so now is the time. The New Onigashima Project is literally a giant moving pirate stronghold.
In mythology, this creature was once a man called Taira no Masakado. Here is the origin of the Gashadokuro from Wikipedia: “The government in Kyoto acknowledged his (Masakado’s) attacks as a revolt and put a bounty on his head. Taira no Sadamori, his cousin, and Fujiwara no Hidesato killed him in 940, decapitating him and brought his head to the capital in Kyoto for a reward. Masakado’s daughter, Takiyasha Hime, a famous and powerful sorceress, was infuriated at her father’s killers for disrespecting him. She conjured up the first Gashadokuro with the bones of those dead in the battle where Masakado died.”
So the Gashadokuro was born when a skull was moved to the capital of Japan. Sound familiar?
The New Onigashima Project:
I think that Kaido has been lying about the Project and about what will happen when Onigashima touches down in the Flower Capital. He claims it will become a perfect pirate stronghold, but how does that make any sense? The Flower Capital is inland and doesn’t even have a port! My explanation is that Wano will be torn apart as the giant skeleton breaks out of the ground, and it will instead become a moving stronghold. From there Kaido intends to lead his global war campaign.
Wano is notably an extremely tall country (I believe the tallest in the series). If the Gashadokuro does indeed exist on Wano, I suspect that it was sealed there at the end of the Void century and its head was relocated to the island of Onigashima in order to deactivate it. We know that the people of Wano were heavily involved in the documentation of the Ancient Weapons (via the poneglyphs), so wouldnt it make sense that one of them actually ended up in Wano?
Some of the sections of the Skull are actually named after parts of the brain, such as the Right Brain Tower and Left Brain Towere. This implies that the Skull is a real skull. Additionally the actual panel where Kaido mentions using the Ancient Weapons is a panel of the Skull.
Kaido keeps insisting on Wano’s importance, and he sure isn’t there for the oshiruko. Yes, he seems to be interested in history, with his apparent knowledge of Joy Boy and Wano’s importance, but this may all relate back to Uranus. After all, the people of Wano are the creators of the Poneglyphs which are heavily tied to the Ancient Weapons.
Kaido has built his castle directly inside the Skull, which would be the perfect place to control Uranus from upon its activation. It would essentially be a giant unstoppable skeleton mech.
*Theory by TomatoHurk