Shanks is probably the Yonko that has the least endgame goals of all the Yonko.
One possible endgame goal for Shanks might be that he is ‘guarding’ Raftel and the ‘One Piece’ until a suitable successor of Roger comes into play. It is possible that Shanks is blocking anyone from nearing Raftel, and he might even be guarding the ‘lost’ Road Poneglyph. If this is his endgame goal, it would explain why he isn’t actively pursuing anything except for having a good time, while still making sure that he and his crew are strong enough to compete with any other pirate crew in the world.
*by L.