User blog comment:Kaido King of the Beasts/Chapter 822: The Big (Plot) Jump/@comment-24850639-20160407123544/@comment-3185063-20160407171206

I think that island at the end of chapter 821 could be Kaido's "favourite island", although, the trees suggest otherwise that it could be Wano.

Edit: I have since re-read some of the manga, and, in Chapter 793, we see the same island, with the same trees and the same figure in the background. The way the manga refers to it as "A Certain Island" in both instances is similar to how the Kid Alliance's hideout/Bege's location were referred to in the same manner, as "A Certain Sea", Urouge in "A Certain Sky", and Bonney as "A Certain Sky".