Just realized something after seeing Kaido again, anyone else suddenly remembers the.. umm. Cow King / Gyumao / or whatever the hell the name is? U know, the Cow King from Journey to the West. It's probably due to that huge horn of his and that giant club/mace.
And if this is true, that Kaido is based on the Cow King, it won't be far-fetched to think that Kaido n Luffy will end up as friends. Cause he n Songoku was on a pretty good terms in the Journey to the West story; and in a lot of iterations I've seen, the cow always ended up in good terms with the monkey (in DB, he even became Goku's father-in-law).
I know that even if Oda based Kaido from the Cow King, he wouldn't necessarily follow the original lore, just throwing out ideas here.