Long ago, Shanks put his trust in Luffy to fulfill Roger’s dream of creating a free world, but just overthrowing the World Government and dragging down the Celestial Dragons, like the Revolutionary Army is doing, is not enough. There needs to be a plan in place to replace them. A plan that is permanent.
So basically, Shanks knows about the Void Century, he knows that anyone who knows about it is in danger of being wiped off the map by the World Government, so he keeps the secret, waiting for Luffy to become the Pirate King, while trying to keep the balance of power so that neither the World Government, nor any Yonko, can prevent Luffy from fulfilling the will of D.
That is honestly what I think Shanks’ place is in the story. He’s an intermediator, a temporary peacekeeper, the only one who has been able to keep the balance intact between the great powers, keeping war from tearing apart the innocent while waiting for Roger’s chosen one.
This is also why I think Blackbeard will kill Shanks.
It will make perfect sense. Blackbeard will start the greatest war ever, not Kaido, nor Luffy or Monkey D. Dragon. When the balance pin known as Shanks falls, that’s when the world will descend into chaos.
After that, Luffy and his crew are left to deal with the situation in which everyone tries to eliminate the other, including Blackbeard, the Revolutionary Army and the World Government, all clashing at once. It’s gonna be crazy!
Picture it, Shanks laying dying in Luffy’s arms, smiling, saying “you were the one Roger was waiting for…all along”.
*Theory by Freshairkaboom