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.
I don't think their fight will be to the death, at all, but I don't see Shanks not fighting Luffy just to see how strong Luffy is and whether he deserves to be a pirate king. It's a lot like Roshi's intentions for Krillin and Goku when he joined the tournament - to make sure they don't get complacent and to see how strong they are. As you said, it can be a spar to see how strong Luffy is, but I still think they'll somehow fight. There's no reason for them not to, especially when Shanks would likely stand in Luffy's way even as ally since he's a yonkou. Hell, Shanks could be the mini-boss while Blackbeard is the boss of the arc or fight with Yonkou.
The crew fights coordinated well, but the Yonkou fights will be like CP-9. The crew will be against strong opponents that will prevent teaming up. The Yonkou will be like Lucci at the time, only Luffy stands a chance. Big Mama and Teach he will want to beat because he does not like them. Kaido unless given a reason will be fought against like Shanks. In Luffy's mind the pirate king has to be the strongest. he would have to win in a spar against Shanks to prove he is worthy to be Pirate King. I do not see that battle ever happening against Shanks. There battle would be short maybe a one punch each and Luffy returning the hat.
It will more than likely be 1v1 because Luffy has to be the one to take down the captain of another crew when they are strong. If it takes his crew to defeat a captain, then he can't claim the title of a Yonkou because it took his entire crew. When getting that title, you are saying that it has to be a captain vs. captain in that situation as they are the one that is recognized as the Yonkou. Not their crew, but the captain.
I further think that because it fighting Moria who was known to have once fought equally agianst Kaido (I'm thinking it was before Kaido grew to be a Yonkou), Luffy was the one to take out Moria. So it has to be a captain taking out another captain.
When you cross paths with a beast, are there any guarantees... that you won't be mauled by it