Do Shanks knew about the One Piece already because he was boarding the Pirate King's Ship before? If it yes, why he still go pirating and even became a Yonkou,? Knowing that someone, and he knew that Luffy is the next Pirate King. Do you think all the pirates and Yonkous know about the One Piece already in the New World? So they made territories to control or take over the New World instead.
I don't think he's after One Piece. Maybe he just wants to conquer the seas or probably like Luffy, have the most freedom.
I think Shanks' goal is to be sort of a guide to new age. He believes that Luffy can do what Roger couldn't. That he's the one Roger was waiting for to fight against WG. And he will help him do that, even if he has to fight him to make him stronger. Well, that's how I see it.
(Btw, you didn't need that apostrophe in the title.)
I think his goal is just to party and get drunk all the time.
What Shanks is after?
-Revenge for what Blackbeard did to his face and Beers