While I can definitely see Kaido dying in the end, I also think that Oda will throw a curve ball at us. My theory is:
Kaido will be beheaded, but that doesn't kill him.
I would argue that Kaido's decapitation was foreshadowed even more often than his death. Sure, you could say that it is one and the same, but Orochi begs to differ. And even Doflamingo seemed to have been beheaded in Dressrosa - it was just a clone, but two of Kaido's closest allies seemingly being decapitated could be foreshadowing that it will be the same for the big bad of that entire storyline.
There is also a lot of similarities between Kaido and the
Myth of Shuten-doji, the most important one for my claim of course being
They cut off the drunken Shuten dōji's head, but even after cutting it off, the head continued to bite at Minamoto no Yorimitsu
And Kaido being immortal isn't a completely new theory, whether if it was done by the previous user of Law's devil fruit or is part of the powers of Kaido's own devil fruit, I wouldn't say that theory is dead.
Another reason for me to believe that Kaido will not die - although a significantly smaller one - is the screentime compared to Big Mom. She had an arc before Wano, is very present in Wano, and might even get more after Wano if she plays a role in Elbaf. Which is pretty likely considering her flashback and even smaller points like Lola being back via the coverstory. Kaido only having one arc, albeit the biggest one yet and maybe ever, seems unfair, and the fact that Big Mom is also part of that same arc doesn't make it better.
As for what happens to Kaido when he's basically just a head... well, the alliance with Big Mom might still persist afterwards, but I could also see Blackbeard taking the head, as Kaido has knowledge about Joy Boy, but more importantly about Rocks, and the connection between him and Blackbeard is more than evident.
No matter what, Kaido's head would go to Elbaf somehow, which seems to have strong influences from Norse Mythology. And guess who Kaido would be?
Mimir, the keeper of the well of wisdom and also "just a head".
...I know that this theory is pretty extravagant, has many "ifs" that would need to happen and thus the likelihood is definitely less than five percent. But I like it, not just because it is mine and I haven't seen it done anywhere else, but also because it ties a few other theories together while still being new.
And I know a lot of people who would hate for Kaido to be reduced to a talking head, so seeing their frustration would be a plus as well.