If it does happen, it'll happen likely after Luffy wins with a final blow but who cares. Its still a huge moment if it does.
Tbh, taking down Kaido (and maybe Linlin) SHOULD be a combined effort. These characters are not strong enough to handle a 1v1 alone with these 2 and still have a variety of other strong opponents to deal with after Wano.
Oda is making Kaido, the world's strongest, go down first because he likely wanted to employ an "Oars-type" fight to the mix. Blackbeard/Shanks/Akainu/Im may all be 1v1 fights with Luffy in the end, so it makes sense that this happens.
Zoro actually beheading Kaido is symbolic because it IS what Ryuuma did hundreds of years prior, and we will probably learn that they are connected via the Shimotsuki clan in 10-20 chapters anyway.
No johns to Sanji fans, this is just a plot device that has been set up by Oda. Im actually more interested to see how Big Mom factors into all of this or how this Rocks plot is gonna grow because that has also been set up by Oda.
I think more players are showing up on mainland Wano soon (former Rocks pirates, former WB pirates, Big Mom Pirates, etc). Kaido wasn't a "world of violence" so this is how you achieve it. I think this war is gonna be nuts