Honestly outside of enma tribute i don't understand what king of hell means to zoro
If its just because of enma or him willing to be a monster or demon for luffy like half the strawhats i think itd be lame but idk
Maybe its something else that zoro will flesh out later as to why
It's probably related to why he can use Asura and what the Grim Reaper is too.
Sanji and Zoro have both associated themselves with Hell and demons, Robin was the demon child and has a new demonic form, I think Nami was called a demon witch before amd Franky was referred to as demonic as some point?
Brook was confused for Satan himself by cultists over the TS and rock music I seen as Satanic IRL, plus Brook has that sort of demonic soul vibe with what he did at WCI too, with the huge spirit guitar attack.
Chopper willingly became a Monster for the SHs too and I'm sure we don't know the full name of his DF yet either, which is important.
Oda has made demons and Hell a recurring theme for the SHs and Zoro wanting to be the King of Hell seems to play up on this ofc and as someone else just said, parallels the Dark King Rayleigh too.
Wano is referred to as Hell too and Oden had a sword specifically named Enja bade for him too, very interesting and bringing this full circle too...
I have a feeling Mihawk will have some Hell symbolism and maybe he or Shanks will have their own version of Asura too.
Ironically we have another Asura/Ashura who has Shutenmaru and Doji in their name and named after the King of Oni Shuten Doji, who Kaido is definitely majorly inspired by too. Especially even more after his new attack names and backstory now.
On a side note, Kaidou seems to be his full name only which is disappointing, not even Kai D Ou or Kai D Oars or something at least.
So much for Kaidou being related to anyone we know, especially a certain Marimo, Worororonoa Wororozoro.
I still feel Elbaf will flesh out the ancient giants (Oni) more as Oda keeps referencing multiple freezing cold/winter islands and not elaborating on them.
If you really want to go crack conspiracy theorist too, there's a guy in Impel Downs level 5 freezing hell, rumoured to be Xebec but renamed as some fodder prisoners claim he's "preserved for the future" and Imu has a frozen crypt/tomb too.
I even believe there are Frost Giants in OP and that Imu may be one. Ymir even?
In Norse mythology, Ymir, also called Aurgelmir, Brimir, or Bláinn, is the ancestor of all jötnar.