Perhaps, now with the economy working in continous functions rather than spontanious, coring could also be a continous function
I.e a nation may choose to core a city, and instead of it happening instantaneously, it cores over a time period. Costing gold until it is done, maybe a nation may choose to stop coring if they are short on gold, then continue coring at a later date
And a partially cored city may have certain effects
- a 50% cored city would need 150% turmoil before revolting, 72% would need 172% etc
- any city below 33% cored costs 1 gold a tick, 33% - 66% has a net zero effect on gold, over 66% cored gives 1 gold a tick
Just an idea