In Chapter 968 it is revealed what Roger and Kozuki Oden heard from the Sea Kings during their trip to Fish-Man Island.
It is shown to be a discussion between two sea kings about the birth of the new Poseidon and her meeting with another sovereign who is to be born in a distant sea. The sea kings knew that the new Poseidon would be born in 10 years and reckoned she would need 15 years to grow.
So one king is Shirahoshi but who is the other one? I bet it’s Luffy.
Luffy was born in a distant sea (East Blue).
This means that Luffy and Shirahoshi were destined to meet. Luffy is the successor of Joy Boy and Shirahoshi is the successor of the original Poseidon. Perhaps Luffy and Shirahoshi are the reincarnations of the souls of original Joy Boy and Poseidon.
So Joy Boy and Poseidon are the names of the first 2 Kings during the Void Century to fight the 20 Kingdoms? Then the names became titles, for Poseidon it’s anyone who possesses the power to command Sea Kings while for Joy Boy it’s anyone who inherits his will.
“The whales are delighted in anticipation of the day the two sovereigns shall meet again”. Well, it’s already shown in the previous arc.
The whales seems so happy when Luffy finally reached the New World and this happen when Luffy and Shirahoshi already met and became friends.