There is a popular theory which states that One Piece is a way to gather and use the Ancient Weapons to destroy the Red Line at Reverse Mountain and Mary Geoise, connecting all Four Oceans into One Piece – fulfilling Madam Shyarly’s prophecy, and a lot of the Straw Hat Pirates’ dreams too (e.g. Sanji’s dream of finding all Blue).
I think this is the case because of the recent references to Poseidon, with Roger mentioning he was too early to do it because Shirahoshi hadn’t been born yet.
The phrase in particular that I think supports this is the passage about ‘making the whales happy.’
The formation of the Red Line after the great war decimated the whales population. Baby whales are caught in Reverse Mountain’s stream and sent to the Grand Line with no way of returning, seperated from their pods (like Laboon).
Also, migratory routes were blocked. Whales are seen around Fish-Man Island also, as they are trying to migrate but get stuck so they hang around there. The destruction of Reverse Mountain will allow these kids to reunite, and restore the whales’ migratory routes to normal. I think the Pirate King and The Sea Kings are to work together to destroy it.
Maybe Joy Boy designed all the Ancient Weapons with the sole purpose of helping the whales, that might be what Roger was laughing at.
*Theory by seedyProfessor