Now then, what does Shanks really want? How does Yonko come into play, what does he want from the World Government and more importantly, what about Luffy?
Shanks wants to be the only Emperor to rule the New World. Yes, you heard it right. He wants to be the Emperor, but not the “Pirate King.” He wants co-existence with the World Government. He knows that World Government does it’s all to prevent war and isn’t blood-thirsty (mostly). Yonko on the other hand, are aggressive. Teach and Big Mom openly declared that they wanted to become the Pirate King. Kaido wants the whole world to go into a huge war. Big Mom tried to murder the whole Vinsmoke Family to become the Pirate King. Teach killed a Yonko for it.
Out of all of the current and past Yonkos that we know of, only Whitebeard was a peaceful Emperor (aside from Shanks himself), he wanted to keep the status quo—peace—intact. And that’s why Shanks gave him a proper burial.
Shanks met with Gorôsei to talk about a certain pirate during the Reverie. I think the person of interest there was Blackbeard. Because, Shanks wants to beat all the other Emperors and Blackbeard is comparatively the weakest among the Yonko. World Government isn’t stupid, they want peace to be held up, so they heed Shanks’ words. In my opinion, Shanks wants the support of the World Government for his co-existence plan. With help from Marines, he would beat Blackbeard and then Kaido and Big Mom, becoming the Emperor of the New World, creating a world without conflict without needing to find One Piece.
But Shanks gave Luffy his hat? Exactly. He gave Luffy his hat and wanted Luffy to reach him and be a great pirate. Shanks never told Luffy, exclusively, to become “The Pirate King.” He wants the Future Pirate King candidate, the most likely one in his eyes, to not be a Pirate King, but be an underling or an equal and rule the New World. He doesn’t want Pandora’s Box to be opened. He doesn’t want the war that will come with One Piece. He only wants peace, and he will do whatever it takes to achieve it.
From a narrative standpoint, this will create a huge conflict between Luffy and Shanks. Luffy will have to choose between listening to his well-reasoned arguments and stay an Emperor and abandon his dream of becoming the Pirate King OR fighting his mentor for the title.
*Theory by LTLoefer