It was a good thing AEG ultimately decided to place Theah and Rokugan in separate worlds, rather than merging them as they apparently originally intended to, with "Cathay" originally being just another name for Rokugan. The technological and social disparities between the two ALONE would have had rather unfortunate implications, with Rokugan being hilariously technologically backwards by comparison (largely due to deliberate self-sabotage, canonically). Cathay may have been an... imperfect supplement for 7th Sea, but it at least portrayed an eastern nation that was easily on par with the rest of Theah technologically, while the Burning Sands supplement for L5R meshes a lot better with Rokugan's metaphysics.
Also I have NO clue how they originally intended to explain the Great Fire Wall, given that if you look at where it's supposed to be it would be in Unicorn, Dragon, and Phoenix Clan territory in Rokugan, I think, but it's never mentioned in any of the Rokugan books.
I am basing my assumption that they were originally meant to be in the same world by some characters from Theah that had some crossover in early l5r books, and the fact that Theah's "First Prophet" of the Vaticine Church bares a striking resemblance to Shinsei/Mekhem in terms of personality and teachings.
