What if Shanks does not want to fight Luffy? It's a situation that does not need fighting to be resolved, and can come to a more satisfying conclusion if they don't. With Blackbeard the only major opponent left, Luffy and Shanks reunite. Shanks acknowledges Luffy as a "great pirate", and recognizes him as future pirate king. He agrees to become an ally of Luffy, symbolically abdicating his own position as Yonkou (even if that position has lost all real value by that time). Shanks then promises to help Luffy defeat the Blackbeard Pirates, mutual enemies. With that done, he helps lead the way to Raftel. That would be much more satisfying, and far more within plot boundaries, than Luffy having to defeat Shanks in a one on one fight, like he was a common antagonist.
That also would explain what Shanks meant when he said that he
bet his arm on the new age. Like Whitebeard, I do not think he wants to become Pirate King. We don't know if Shanks ever reached Raftel, but he was on Roger's ship. The possibility exists that he already knows how to reach Raftel, and knows the identity of One Piece. Buggy too. There is a strong possibility he already knows that he can not reach Raftel himself, and that One Piece is out of his reach. Hypothetically, if the Will of D and ability to hear the voice of all things are needed to reach Raftel, Shanks already knows that he himself can not be Pirate King. That is why he made a bet on Luffy. Though a child at the time, he was still a D, and Shanks might have seen true spirit in him. That would explain why he gave Roger's strawhat to Luffy, why he sacrificed his arm, and why he waits for Luffy to become a great pirate. Buggy may know the same, the reason why he has shown more interest in treasure hunting than becoming pirate king. He might already know Raftel is out of his reach, and that One Piece is not gold and jewels anyway.
At best I think they spar, Shanks testing out Luffy's strength. Nothing serious though.
