Random ideas on China:
UP - Mandate of Heaven: Reaching a new era grants a temporary maintenance/upkeep bonus.
Alternatively, for more of a challenge: Being first to reach a new era grants a temporary maintenance/upkeep bonus (Though this might be too self-reinforcing, which is bad for China).
This bonus would be temporary because it would decay by 5% every X turns until going back to 0%.
Reasoning: The current UP is fine (though it's hard to use it to its full potential given how fast you reach your happiness limits), but I've seen many attempts at a Mandate of Heaven-themed UP so I thought I'd try too. The big problem is that this is a concept that mostly has to do with internal conflicts, which aren't really represented in DoC outside of the abstraction of stability. But a chinese stability boost wouldn't be terribly interesting since 1) Human China doesn't have much stability troubles in my experience 2) AI China should have stability troubles and collapse repeatedly and 3) a stability-focused UP doesn't sound that fun for a civ that does very little conquering. If stability is out, maintenance/upkeep is the next most thematically appropriate thing, and it's certainly useful for China. The decaying part I think also hints at the cyclical nature of Chinese history without going overboard with forced collapses, etc.
UHV 3: Start a golden age in four different eras before 1800 AD.
Reasoning: The basic idea is to put more pressure on the player's choices between bulbing techs vs triggering Golden Ages. If the player fails a Golden Age in one of the earlier eras (Ancient, Classical, Medieval, Renaissance), they'll have to scramble to reach Industrial before 1800 AD, which seems like a nice alternate history goal and would make the post UHV 1 & 2 game more interesting.