So, about Kaido. I have read a few theories on what exactly he is and how he will die.
All those theories kinda seemes plausible but were missing the greater picture. I will try to make it short:
1. What is Kaido?
Kaido is an immortal Dragon, a deity. He got bored of his immortal life and wanted to have adventures. That’s why he ate a ONI-Zoan making him “a human” in a way. In this form, he could finally go on real adventures – by becoming a pirate.
He realized that he still had his immortality by being captured and commiting suicide attempts to understand if he really is immune to pyhsical damage. He did never drink poinson or jump into the sea, because he did not want to die, he wanted to see how immune and strong he is. That made him destructive and confident.
2. Why is Kaido in Wano?
The only time he got hurt really bad was before he even ate the fruit, and it was by Ryuma, the legendary dragon slayer, who had the sword – Shusui. So once Kaido became Human, he realized that the only thing able to hurt or kill him, are either samurai of noble bloodline (Kings of Wano) or one particiular Sword, Shusui.
After finding out that Shusui was stolen 23 years ago, Kaido got scared that someone knew his weakness. And he was right, it was Moriah, stealing Shusui and Ryuma to use them as payback for Kaido killing all his men.
So Kaido came to Wano and searched for Shusui and the thiefs for 3 years. He could not find the sword, but he figured that someone on the Island had it. So he overthrew the Kozukis and tried to eliminate them to make sure, that as few people as possible know about Ryuma and Shusui and all that do, obey his orders.
3. How to kill Kaido?
It will happen by the hands of Shusui’s current holder Zoro or Luffy (who picks it up while all the Straw Hats are almost done for as a last resort). Not because Zoro or Luffy will be stronger or anything. It will be that Kaido will feel confident that none of those Rookies and others can come and hurt him anyways (which they can’t) – apart from Shusui.
Oda always shows the Straw Hats are not the strongest crew but a good team and with a lot of willpower and creativity. So Kaido will be the first Yonko to really fall or die.
*Theory by likkaalien