this is my first post, so feel free to let me know improvements.
- Kaido is said to be immortal.
- The Ope Ope no Mi gives the user the ability to do a special operation: the fruit user can make someone else immortal by trading the user's life.
- We know the story how Law becomes the Ope Ope user.
First I thought that Law will make use of his special power to rescue Luffy; in a way his "thank you" for Luffy helping him with Doflamingo.
But here is another possibility:
I claim Kaido's immortality is due to the power of the Ope Ope.
Here is some justification:
Why was everyone after the Ope fruit back then in the backstory of Law?
This suggests to me that the devil must have returned into a fruit, again.
And this, because the previous user (probably a well known) has passed... maybe by doing the special operation on Kaido.
This of course would be an explanation why Kaido cannot succeed in his strange "hobby".
And Law might be the only one to remove the immortality by doing a "reverse special operation".
Let's hope that the sacrifice can be tricked...