So far, this seems to be one of the major distinctions between a regular Yonko and the Legendary Pirates.
Confirmed Conqueror’s Haki users among Roger Pirates:
Confirmed Conqueror’s Haki users among Xebec’ Crew:
Confirmed Conqueror’s Haki users among Whitebeard Pirates:
All of the legendary pirates, the greatest of the greatest captains have had multiple Conqueror’s users beneath them. Meanwhile the regular Yonko have only had at most one.
As Luffy is about to take the next step into becoming Pirate King, it is simply essential for him to become a Conqueror of not just one Conqueror, but of multiple. To sum it up, Luffy needs Yamato in order to be Pirate King.
Now I know what you are about to say. What about Sanji? To which I must respond, what about him? Sanji isn’t a Conqueror and Oda has never given him the same Conqueror portrayal that Luffy and Zoro have. He has a strong will, sure, but so does Law, so do the Admirals. Ultimately Luffy to be Pirate King needs something more than what he currently has.
Like look at the Roger Pirates. There is a clear parallel between the Roger Pirates and the Straw Hat Pirates.
Luffy – Roger
Zoro – Rayleigh
Sanji – Scopper
Jinbe – Sunbell
Sanji is Scopper, a longtime member of the crew and Zoro’s rival.
The way Scopper is portrayed here, alongside Rayleigh, shows that he was the Sanji of the Roger Pirates.
However, Roger needed more than Scopper to become Pirate King and Luffy will need more than Sanji. Fundamentally, it was the addition of Oden into the Roger Pirates that solidified the Roger Pirates as a legendary crew.
Look at it. Roger and Whitebeard were close in power. Then Oden, Whitebeard’s strongest man, leaves Whitebeard for Roger. This makes it to where the Roger Pirates with Oden are undeniably the strongest crew at the time.
I think Luffy needs his version of Oden. An immensely powerful outsider who joins the crew late in his journey and ultimately plays a major role in the quest for One Piece. There is no better person for this position than Yamato. A person who not only expressed their desire to join, but who was also accepted by Luffy.
As for why Yamato didn’t join in Wano, the answer is simple. Post Wano, Luffy was supposed to be just a Yonko, not a Legendary Pirate. Yamato joining would overpower the crew, much as Jinbe joining in Fishman Island would have overpowered the crew. Jinbe was a 3rd Commander, a position that solidified Luffy’s status as a Yonko. Yamato as a second Conqueror would solidify Luffy’s position as a Pirate King.
One final point. Blackbeard clearly has his own version of Oden on his crew in the form of Kuzan. I’m not saying that there is any sort of parallel between Oden and Kuzan, however they both occupy a similar role in their respective crews. Kuzan, like Oden, is an overpowered outsider who joined late in the crew’s journey. Much like how Oden separated Roger from the rest of the great pirates of his generation, Kuzan separates Teach from the other Yonko. No other Yonko crew has, in addition to the captain and the first mate, someone as powerful as Kuzan.
In order to match Kuzan on the Blackbeard Pirates, I believe that Luffy will need Yamato by his side in the final clash between the Straw Hat Pirates and the Blackbeard Pirates.
*by Monkey D Theories