On a different topic, I've been playing China a few times. It might need some kind of Three Kingdoms event, because playing up until the Mongols is a bit too calm. The barbarian and independent cities & units that spawn add some variety but I think it might need a bit more to do. It would also make the later waves of East Asian spawns feel more like the post-Roman European spawns, filling a void left by a divided China. Naturally, this might affect the UHV since China would be more difficult.
I don't think this would require adding another civ; you could achieve it by splitting China into Shu, Wu and Wei, where the player gets one part, Indy1 gets the second, and Indy2 gets the third. Player might be able to choose which splinter nation to play (i.e. they don't lose those cities to Indy1 or Indy2) via a pop-up. If China is AI-controlled, default to Wei, since it is in the Chinese core.
This could probably be preceded by Yellow Turban style uprisings of "barbarian" units.
For fun I might see what kind of changes I can make in scripting to support these events, how much work they would represent and what the effect on gameplay would be.