This thread looks like a good place for my theory.
I believe there's a good chance King is Uranus.
"Road to Laughtale" Vol.3 had a page about Joy Boy, summing everything about his connection to the ancient weapons. That's where I came up with the idea that maybe each weapon needs a trio: a controlling person of some race, huge beasts connected to that race and a huge ship. The secret legend of Joy Boy must be passed down or otherwise reach this era from 800 years ago.
Poseidon is the god of the sea, and we have Shirahoshi, sea kings and the ship Noah. Joy Boy's legend was passed down in her family.
Pluton is the god of underworld, it is in Wano, where there's a volcano (didn't Kaido and Big Mom fall straight into Hell?) The ship resides beneath ancient sunken Wano. The controlling person is Momo and the beast is Zunesha (could be the Minks too). There was no need for Zunesha to pass down the legend as he is still alive.
Uranus is the god of the sky. The only other person who knows the secret legend of Joy Boy (other than Roger and Oden, who learned it in Laugh Tale from the story of Joy Boy himself) is King. It was King who told Kaido about Joy Boy, mistaking him at first to be the one.
Kaido declared he planned to lay his hand on all three ancient weapons. Why would he declare that openly if he didn't think he already had one, or even two?
I've been told that Enel has a ship, and Gunfall has a big bird. But none of them had the legend passed down to them. The legend with Joy Boy's name only passes down in the families of ancient wepons. Besides, Enel's ship was new, and there are many big birds in one piece (including the one at the very beginning who caught luffy in it's beak when he tried to hunt it).
So I believe King is the anceint weapon Uranus and his story is merely beginning.