I don't think GWs are unique to any one empire. Besides, times change, and an additional civ that wants/needs Great Works to flourish isn't going to topple the cart.Wu's dynasty, the Tang dynasty, oversaw the greatest period of Chinese poetry and art - to skip over that was (and is), in my opinion, quite sad. Now we have a glimpse of that.
Yeah, but that's what you said before IIRC. Though yeah, times change so it's not an important point, though greatest period of art and poetry point you sort-of defeat that point by saying that great works are not unique to any one empire. But that's not relevant.
It is also France's niche (conquest), and Assyria, and the Zulu, and Carthage...which is to say that settling and conquering is no one's 'niche.' It is a core gameplay mechanic. The fact that China benefits almost exclusively from overexpansion in the first 1/4 of the game (GWs are hard to come by before medieval) puts you in a tricky spot. If you, theoretically, settle 10 cities before classical (which, in my opinion, sounds like a farcical number), then not only is your infrastructure hurting, but you are overextended and your policy costs are through the roof. You can recover, but it is by no means guaranteed.
China's an expanionist snowball civ. If you don't contain the Middle Kingdom, it'll continue to expand and will dominate.
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I didn't say before classical, I said classical which is pretty much always doable. It doesn't seem tricky at all, too, since the bonus Food + WLTKDs should speed up your city growth so you can work Production tiles much faster. You can always just go Authority to alleviate any flaws (lots of Production + can get through the tree very fast with tons of UA culture), though at the cost of Great Works. Policy costs "going through the roof" doesn't matter, everyone suffers the same issue to a far greater extent, it is China alone that alleviates it with the UA which means another city will always be viable Culturally, and the bonus food will allow it to work specialists/mines to get lots of Production. Another thing that combats your claim of them benefitting almost exclusively from overexpansion is the fact that China will not only grow like mad allowing you to work all the Production tiles you want, it also has an early game UB that already is decent (not top tier, but above Basilica/Mission tier UBs) which will further result in the snowball getting harsher. Everyone has this sort of a problem, but this civ would get it solved by the UA instantly. Like yeah, sure, you might start in the middle of a desert and/or with no production tiles at all or something, but in that case just imagine how hard it would be for any other civ.
This China is going to have no weaknesses whatsoever. It starts with the strongest base effect (+1F+1C +WLTKD turn 1 on capital) that by itself gives you benefits comparable or superior to what an early pantheon can grant India at this point, the effect only grows stronger with more cities and great works. No period of weakness. Every civ has one.
Sejong balls out of control with time but he starts with little to nothing, Rome's ability only matters on conquest for further cities, and even then won't compare and it requires lots of Colosseums, Spain only gets meh bursts of yields and needs to get to medieval to get its best stuff, Poland only benefits after getting XX techs, Denmark needs Sailing/Fishing, likes water maps and needs Jelling Stones everywhere, Egypt needs access to a wonder/Great Work, Maya/Babylon/etc need techs, Aztecs need kills and the UA falls off though Floating Gardens do not, Austria actually needs to be an ally for 50 billion years and have gold, even Polynesia actually needs tiles to settle their Ankh Morporks as well as time to construct them, etc.
Meanwhile, China starts with the best base effect (unless you go god of all creation as India but why) and as you expand and get GWs, the UA also remains best or one of the best. Can anything really compare with that? Sure, some UA might perhaps get very, very slightly more by renaissance, but by that time China has more policies than Poland and more pop than any other two civs put together (excl. India/Monty). I am absolutely certain it'll be broken. Snowball starting from turn 1 that only gets stronger and worse with more turns and cities as well as buffed early game UB like paper maker just cannot result in something balanced. Even without paper maker it'd be too strong.
That's easy, scale or limit based on map size.
How? Only tech/culture cost per city scales based on map size. Some civs/trees are better on bigger maps, some on smaller, none IIRC are balanced by map size/map type - making an exception just for one civ would definitely spark discussions like "nerf Progress on Huge (playing on Huge is HUGERESY if you know what I mean, I play Large or Standard myself)", "nerf Polynesia on archipelago", "nerf Iroquois/Celts on forest maps", "buff Tradition on any map size higher than standard". It's just tons of work and some will always complain as it'd be very hard to balance. How to determine if something is too much? What if you play on a Large map, but so much water spawned you actually have less workable tiles than on an average Standard sizer? Too many variables, no way to account for them. It's not game length by which everything already scales as it should.
But yeah, on Large it'd be unbearable. 20 cities is inevitable, and at that point it's 200+ Culture and Food from the cities alone. That and a game-long WLTED. However it's not a big problem because as-is, China doesn't look like it can be balanced on any setting.