Im just quoting both since i can answer both of them at once. To add everything else, i.e. all of the asian continent and water and so forth, we're looking at at least, at the most abstracted and reduced 1000 provinces, out of those maybe 1/5th or so would be water, leaving around 800 or so provinces which need history for all characters and so on. So imagine 800 or so provinces, with history stretching back to 1AD (because thats how far the game can go back) and features and events for everything related to china, new religions and so forth. The workload alone would be immense, but doable, but the engine? I dont think it can handle it, certainly not now and im very sceptical about the claim that it somehow could handle it after the 2.4 patch without going right back to where we are now. Once all that stuff is added, all that optimization went out the window because its now taken up by all the new stuff so we'd be right back where we are now, if not even worse off potentially.
But do you want it abstracted and just barely representable? Why? "Because then its in the game of course". What for? What is the point of adding a subpar region with fluff that barely counts as feature complete just so you can go "Yay we got Quasi-China in the Game now! Lets add some more shit because we will keep asking for more anyways"? Because thats whats going to happen, it doesnt matter whether you will do it, or someone else will, eventually people will ask for even more. See Africa, maybe we should add that too next because its also part of the "Old World", sure south africa has barely any influence on much of Europe, but China didnt really either so again, impasse in the argument. Why should we accept a barely functional DLC just because you want to point to it and say "There it is!"? Go look at a damn map then, it would have about the same effect.
The argument circles always back around to what a few ask for, never considering anything else other than having their pet-desires added to the game. I wouldnt care if China was added unless it actually offers a unique and fresh experience and honestly, i dont see that happening. So for the sake of fairness, i will be Devil's Advocate and just tell you about all the things that it will impact. Like performance, because the Engine will eventually hit the ceiling and cant get any further than that, no amount of "but but optimization" will change that. And adding everything else that hasnt been added yet isnt going to happen under the current engine, so they'd have to essentially do an engine swap and THAT is a fuckton of work.
You know, i will make you a deal. Once 2.4 hits and the performance gets better, lets see about getting Umbra Spherea another shot with that patch, lets see how well it runs. If it runs well, we can talk about whether it would be feasable to do an entire DLC on all that stuff. But im not going to hold my breath and neither should you, lack of air is bad.