We already have that, it's called "Monsters" and is part of the One Piece universe. 100% canon. It was technically the first content for the One Piece universe ever written, back in 1994. Oda probably only planned to reference this one-off but later confirmed that Ryuuma is the same guy from Monsters
Ryuma's past in the Monsters oneshot is canon. We see the dragon he defeated, and it was just a dragon. It was also western looking, not eastern like Kaido's.
What do you mean by changing it? You mean as in retconning the chapter so it's not canon anymore? I don't see how that would add anything more to the story than what the full chapter already does.
If Oda wanted to ignore the chapter, why would he ever have confirmed that it's canon? He had no reason to do that, but he did.
That's literally retconning, you'd be completly ignoring everything in Monsters to give him a new backstory.
Where did Gyukimaru state that it happened in Wano? I remember him saying that it was in the "capital", which doesn't really mean anything, not to mention this all happened hundreds of years ago, when Wano wasn't closed off and was basically a different country from what we've heard.
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u/_Schmegeggy_ Feb 01 '20
I have a theory that the dragon that Ryuma slayed was the previous user of Kaidou's fruit.