...I just don't know if I see Oda going that meta with it, at this point.
Oda isn't Grant Morrison or Alan Moore. Or even Don Rosa or Will Eisner. Oda isn't really playing with the medium, itself. If Luffy was using Gear 5 to do truly world-breaking stuff, like changing the shape of the panels, or hiding in the gutters of the manga to launch sneak attacks on his opponents, I might say the "One Piece Manga" idea had some legs. But, he isn't. Luffy's mostly using Gear 5 to summon whatever random item he thinks up. He's using it to create something from nothing, which is a slightly different theme.
One Piece is less about comic books, as a art form. And more about history, in general. It's about the past influencing the present. Not about how great of a impact fiction can have on the world. Usually.
Now, this could change at any time. Oda could just be waiting to get to the really meta stuff later on in the story. Making sure he has room to escalate things by the end. But, for right now, I don't think that theme has been properly set up. The Treasure being a precognitive manga would feel a little out of place, in my opinion.
Now, I'm not going to claim the theme of stories being important isn't there at all. Because we do have a few examples in the story of how fiction eventually becomes reality. Usopp's lies becoming real is probably the one most people notice. But, also, it seems like there's some evidence that the entire idea of a "Pirate King" was cooked up by Big News Morgans to sell newspapers. But, Roger embraced it, and found a way to use that "fake" title to inspire thousands of others to try and become it. Fiction inspires reality. Kind of like how Star Trek's communicators arguably kind of inspired the invention of modern cell phones.
And, I'll admit, I DO like the idea of Luffy eventually getting "manga-ka" artist powers. What would Luffy becoming a "Complete God" look like? Imagine Luffy going Gear 6...and he just disappears! And then he's in a empty blank white space, with only a drawing table with his friends in some trouble being halfway drawn. And then Luffy has to use his God-powers to draw his friends out of the trouble they're in. Slowly aging as he tries to draw more and more, as a sort of parallel with Oda himself. Drawing drains him of strength, but he doesn't really care...because it's fun! Luffy eventually keeling over at the drawing desk, finally being able to give his friends the endings he wants for them. I like the idea...but I'll admit, it just doesn't fit Luffy. For one...Luffy probably can't draw, in the first place! As much as I like the image...I don't think it fits his character. But, maybe that's the gag? Luffy as a God who's a "bad artist" like Konjuro.
Plus, it seems like Joyboy had some sort of precognitive abilities. If his ability to plan 800 years ago for things that are only starting to happen NOW, is any indication. It's not impossible that Joyboy's predictions could have been written down in the form of a manga. Like a book of Nostradamus's predictions, only in manga form.
But, it's not like Oda's been entirely consistent with stuff like that, either. There's the Germa 66, which were villified by the "Sora Warrior of the Sea" manga. It seems like that was just World Government propaganda.
That's not a example of "Fiction inspiring reality". That's a example of trying to "rewrite history". Kind of like erasing the entire Void Century, in the first place.
Every story in this world has a little piece of truth in them. NOTHING is completely fictional. Not the stories about the Dragons, or the Sky Islands, or Mermaids, or Ninja, or even the comic book villains the Germa 66. But, none of them are the WHOLE truth, either. You have to look at different stories from different angles, and dig a little deeper, if you want to find the whole truth.
So, I think THAT'S the theme Oda is working with. This story isn't about something fictional becoming real. I think this story is about different people trying to rewrite history for their own ends. Different people trying to put their own spin on what happened in the past. Turning some people into heroes, and others into villains. Even though, they're all just...people, with their own individual foibles. Erasing the parts that just "don't fit". Trying to ignore that those events even happened, because it doesn't fit the narrative. As we slowly learn what really happened, ourselves. The characters trying to find the truth, underneath all the stories.
Even Roger was kind of "rewriting history" with his story about "his" Treasure. The Treasure likely isn't REALLY about becoming King of the Pirates. And it really seems like it was more Joyboy's Treasure than Roger's in the first place. Roger just used the Pirate King story in order to influence history the way he wanted.
Think about nearly every single flashback we've gotten in this story. Before the flashback, we learn a story about the featured character. Who they're publicly known as, and stories that get told about them. But, then we get the flashback, and we learn what they're really like and why they're doing this. First, we learn about the story. And then, we find out the reality. The flashback we get completely recontextualizes what we thought we knew about the featured character. One Piece isn't really a story about fiction inspiring reality. It's a story about how the simplified stories we tell about each other obscure the more complex truth. The reality is almost always more complex than the story that gets spread around. But, everyone DOES love a good story. And sometimes a good story can find it's way around to becoming real. Lies aren't necessarily bad, just like how manipulating people isn't necessarily bad, it's more about what you're trying to accomplish and why and how you go about it.
And if that's the story that Oda's trying to tell, I don't think the entire world being a literal manga makes sense. It's a interesting idea, but I don't think it fits this story, in particular. I don't think a manga fits the themes Oda's playing with.
But, I'll admit...*I* might be the one misinterpreting things. Oda could go a LOT more meta, at any time. There's still more One Piece to release. And Oda DOES enjoy keeping certain things close to his chest, until it's time to reveal them. So, I can't say it's impossible for the One Piece World to turn out to be inspired by a manga. But...it just doesn't "feel" right to me, you know? That would feel more self-congratulatory than Oda's usually style. "Oh, MY story is the most important thing in the entire world!" Yeah, I don't know. I just don't feel like that's the way Oda's going to go with it. But, I could be wrong. Time will tell.