The first thing Roger did in his journey with Oden was going to the place where the next Pirate King should go to, Skypiea.
Roger went and inscribed the goal of his journey: “I made it here. I will guide this text to the ends of the earth – Gol D. Roger” as if to say, if you made it here, don’t worry, I completed half of the work and took all the Poneglyphs text to Laugh Tale.
There are 30 Poneglyphs around the world, piecing them together would give the full history, but it would take a long time for someone to find them, that’s why Roger went around the world searching for all the Poneglyphs with Oden, to take them to the last island, Laugh Tale.
Poneglyphs and the Promise
In Laugh Tale, there should be a huge empty Poneglyph stone that Oden inscribed on it all the Poneglyphs they found to make the final Rio Poneglyph that Nico Robin is looking for, containing all the secrets.
One part contained an apology from Joy Boy. On its own, neither Robin nor Oden was able to understand what it means, because there are Poneglyphs before it and Poneglyphs after it that, pieced together, completes the story.
After all of them were pieced in Laugh Tale, Roger and the crew read the full story and laughed.
Roger laughingly says: “Joy Boy… I wish I’d been born in the same era as you” indicating something in his era was incredibly funny to Roger and the crew. This will be the ultimate flashback, the Joy Boy flashback and the start of it all. The Poneglyphs will tell his story being a D. and what it meant in the Ancient Kingdom, why he created the Ancient Weapons, and how to use them. Then ending with his apology to Poseidon for the promise, and the collapse of the Ancient Kingdom.
Oda very intentionally did this, Luffy’s promise IS Joy Boy’s promise, to take Poseidon (and Fishmen by extension) to the outside world. You can even see the emphasis on the promise in the scene as if Oda is saying Luffy MUST fulfill this promise. The difference between the two promises was that for Joy Boy, it was his promise alone, but this time, it’s the Straw Hats Promise.
Joy Boy’s tragedy would be that he either tried to save the world alone or that he was betrayed and lost his crew/people.