Oda’s written himself into a little bit of a corner with Kaido. The man is too dangerous to be left alive after his defeat, and his story is too tangled up in the idea of finding a place to die for death not to play into his ending. But it wouldn’t quite fit the series’ shone atmosphere to have Luffy intentionally and unambiguously execute him onscreen.

But thankfully, Oda’s been setting up a possible solution in recent chapters. First, he emphasised that the fires started by Orochi and Black Maria are still burning and spreading through the castle.

Now, in the latest chapter, he’s established that Onigashima is so full of weapons and explosives that it’s basically an island-sized bomb. Anyone with half a brain can probably guess that the island is being set up to blow. And there’s the answer.

After being soundly beaten by Luffy, a wounded Kaido is left behind on the island as everyone else evacuates (presumably to the top of Mount Fuji). He may even recover enough consciousness to reject an offer of salvation, choosing to go down with his base as the glorious death he was looking for.

Remember, the sky island skydive proved it doesn’t have to be death by battle. The island explodes in a glorious fireworks display over the capital and Kaido is never seen again. Killing a seemingly unkillable character by the massive sky explosion will be so terrible, to the point that we’ll likely never see the end of them.

Theory by Captain__M (https://www.reddit.com/user/Captain__M)