And I've been thinking about that scene where Kaido jumps off the sky island. Hes pretty indestructible, apparently he can't die. My theory is that a previous user of the Ope Ope no mi, Trafalgar Law's fruit that allows him to sacrifice himself to grant one person eternal life, did this to Kaido in the past.
Furthermore, when the big battle in Wano happens, the only way to defeat Kaido will be for Law to sacrifice his own life to remove this power from Kaido.
Does this theory hold water? or has kaido's indestructibility already been explained?