Although the fight between Yonko vs Supernovas is starting in a 2 vs 5 right now, I think that Luffy will ultimately end up fighting Kaido in a one on one.
Now let’s talk about the reasons:
1- First of all, before the Wano arc even started, Oda was already hyping up Luffy as a future 5th Emperor of the seas.
This suggest imo that Luffy should come out of the Wano arc as a powerhouse recognized on a similar threat level to that of an Emperor.
2- Before the Wano arc started, Shanks was already suggesting that the time to meet Luffy was close, and that was before Luffy’s Udon training that made him much stronger.
And while we’re still in the Wano arc, Oda implied that Shanks is about to make his move which further suggests the time for their meeting is coming closer, a meeting that Shanks only wanted to happen when Luffy would be recognized as a great pirate.
This could once again only happen if Luffy comes out of the arc recognized as a Yonko level threat.
3- In the One Piece newspaper Vol 2, Oda pointed out that when Kaido was introduced (shortly after Doflamingo’s defeat), he didn’t think Luffy could defeat such a strong character yet.
“When Kaido appeared for the first time, I said that Luffy can’t defeat such a strong character yet. And I’m yet to know how Luffy would defeat Kaido. Probably my audience will not be satisfied if the reason for defeating Kaido is just because Luffy’s punch is so strong. Luffy and I have to find a solution somehow.”