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.
The Final Task
Since Roger knew the exact time Shirahoshi will be born, he gave a final task to some of his crew, one of them was Shanks. In Sabaody, Rayleigh tells Luffy that “I can’t tell you stuff that Shanks didn’t tell you.. but anyways, congratulations for making it this far..”.
Obviously, Rayleigh and Shanks know things that they didn’t tell Luffy, and if you look throughout Luffy’s journey, many things are conveniently placed for him as if to guide him on what to do. It’s like Roger put some of his crewmembers as “filters” for the next Pirate King. There’s Buggy, Crocus, Shanks, and Rayleigh. I included Buggy because I honestly think his role will become much more important to the end of the story.
In Chapter 967, we find that only 1 of the Poneglyphs in Fish-Man Island was missing which is weird if you think about it. If someone wanted to hide the knowledge, they’d take/hide both Poneglyphs. But it’s as if someone wants the next Pirate King to pass, but when it comes to reaching Laugh Tale, they can test him. I bet that person is none than Red Hair Shanks.
There’s a lot of evidence that backs him as the final “block” for the next person to reach Laugh Tale, whether it’s Luffy or Teach.
Rayleigh said they and Ohara were too hasty, and they were. Roger immediately went to Laugh Tale after finding all the Road Poneglyphs. They couldn’t even wait for one of their crew (Buggy) to heal even though he begged them to come, even Oden didn’t look at what happened to Wano, Kaido could’ve already taken over, but all of them wanted to leave quickly. This will be the crossroad between Roger’s crew, and the Straw Hats.
Once Luffy acquires all the Road Poneglyphs, he will parallel Roger’s situation, where a friend’s country on the other side of the world (Oden) is suffering an attack (presuming Kaido already took over) or something bad happening to a friend (like Buggy). Then, Luffy will choose to either go help his friends or go to Laugh Tale “Right Now!”
Luffy taking the other path and not going “Right Now” could nicely link a couple of key things: Buggy coming to the final island with the next Pirate King. And Blackbeard reaching to Laugh Tale first, after fighting Shanks!
P.S: Oda is already hinting something bad happening to Sabo, Vivi, and Hancock. We don’t know how Wano could end, but with the above said, I can see the possibility of the Straw Hats acquiring all Road Poneglyphs by the end of the arc, even the missing one. Then, Luffy will hear about the news in Pangea and face the crossroad.
For Elbaf, I can see Luffy coming back to it after saving his friends, and THEN going to Laugh Tale. Rayleigh told them not to be too hasty and take their time to see the world. Otherwise, they’ll be committing the same mistake.
Finally, Oden said “I don’t know if it’s good or bad luck” about Buggy not reaching Laugh Tale, perhaps his final task from Roger would’ve been different if he witnessed the One Piece, that’s why Oden is questioning his luck(this is pure speculation though).
*Theory by optechhas