Out of excitement for the implications that Luffy may be the prophesied second sovereign and the ‘new Joy Boy’, we didn’t pick up on something so obvious.
Examine the quote: “…of the day the two sovereigns shall meet again.”
The word “again” holds huge significance because it means that Joy Boy was a sovereign i.e. the King of the Ancient Kingdom.
This is something some people have theorised for many years, but this conversation confirms it.
Why Joy Boy? The Fish-Man Island Arc revealed that Joy Boy was a man from the surface in the Void Century that built the Noah and made a promise to Ancient Poseidon.
The Poneglyph we saw in that Arc was revealed to be a letter of apology from Joy Boy, since he wasn’t able to fulfil the promise.
The (failed) promise was that Joy Boy would migrate the inhabitants of Fishman Island into the surface using Poseidon’s ability to control the Sea Kings, who would pull the Noah containing the Fishmen and Merfolk.
When the Sea King in Chapter 968 says, “… and surely all will go well this time” this is in reference to Joy Boy failing to fulfil his promise 800+ years ago.
This confirms that the original second sovereign was Joy Boy and so that he will metaphorically return once again to fulfil the promise — but more importantly that he was King of the Ancient Kingdom.