Since Kaido's introduction !
The dragon scale tatoo he has on his left arm and Momonosuke & Kinemon being traumatised by Punk Hazard's artificial dragons made many people theorize about Kaido being a dragon.
Plus, Zoro beheaded a dragon on Punk Hazard just like Ryuma did in the Monster one shot. Also, Zoro showed some interest in Zombie Ryuma by asking him if he is the famous samurai who slayed a dragon, and Ryuma being tied to Wano as its greatest swordsman ...
Well, there's just so much forshadowing for the ZKK to the point where it's already canon in my eyes