If you happen to get the Abd al-Hazir miniquest and don't portal to town but go to the throne first, he'll say:
"It is strange they have placed Kanai's corpse on the Immortal Throne, as he died before he could be crowned king."
My theory is that Kanai's cube can make a crown which we can place on Kanai's head on the Immortal Throne. The real question is, can we do that now or is that expansion stuff? There's already several hidden recipes, amongst which the Cow level and Greed level, perhaps also a crown. Or they will patch the game during BlizCon and give us all a crown to put on his head.