Just for you guys to be aware, Oda could very well go and say that all these "cut off the head" weren't literal foreshadowing but a thematic elemment because of the feudal Japanese inspiration of Wano. Of course Zoro (or whoever, but if anyone, it should be Zoro) may behead Kaido (I'd find it extremely off in this series, but well), but I seriously don't see why it's so strange for you to see "Japanese" warriors talking about beheading the enemy as if that's remotely unique of Kaido.