I think Shank’s goal is to maintain balance in the world either indefinitely or until Roger’s successor challenges the world to a fight.
I’ve been pondering this for awhile. On the final island, Laugh Tale, the Roger Pirates found the treasure left behind 800 years ago by Joy Boy and learned the truth behind the Void Century, Ancient Weapons, and the meaning of the D.
If the Roger Pirates reached Laugh Tale, learned the history, found the One Piece, and this is so huge that it will shake the world, why are they not doing anything and challenging the world? Well, we kind of have the answer to that already.
Whitebeard alluded that someone will find the One Piece, challenge the world to a fight, flipping it upside down, meaning the One Piece will incite a huge conflict. When asked about why Roger turned himself in, Rayleigh told us that he was dying, and it’s implied that Roger intentionally inspired pirates from all over the world so they will seek out the One Piece and inheirit his will.
I think Roger, after whatever he found, wanted to start that world conflict. But he was gonna die soon. So Roger turned himself in, and the Roger Pirates, are awaiting the day when someone who can finish what Roger wanted to start appears.
Notice how Crocus and Rayleigh are positioned at key points of the Grand Line. It really gives me gatekeeper vibes. Yonkos are the closest, thematically speaking, to Laugh Tale/One Piece. They are the final obstacle for most pirates till they find the One Piece. And I think Shanks specifically is lurking in the New World as a key position. Essentially, they are loosely gatekeeping to make sure the world holds on until when such a worthy successor appears
All of Shanks actions point toward this. “Meet me when you become a great pirate.” Shanks’ four big moves have been, trying to stop Ace and Blackbeard from fighting, halting Kaido, stopping the war, and of course appearing in Mary Geoise to warn Gorosei.