Some more interesting things related to this ORAS/XYZ discussion:
In the Sky Pillar mural
>>56696893 there is a meteor headed to the planet that looks like Deoxy's face rather than a Delta. I'm not sure what that weird barrier of triangles and circles it's smashing through is supposed to represent.
Also interestingly, Rayquaza's body isn't one clean line, and very interestingly each segment of his body even looks sorta like the cells of zygard. The way they link up even sort of matches the kind of malleable and combinable them Zygarde has going on.
Perhaps there was going to be some dope-ass fusion form between Rayquaza and Zygarde to combat the dual primals? Perhaps that's even what Zygarde did in the ancient past, but none of the natives were aware that Zygarde had lent Rayquaza its power?