Here's my suggestion about the recent stability system.
Now the expansion stability is linked with the population of core area, and it's very unfavorable to European colonial civs, especially for Spain and Portugal, these two civs(and most of other European civs, more or less) often has too little core population compared with their other population, I can see Spain and Portugal has a expansion stability at -20+ in about 1700ad almost every time(and sometimes for other European civs). And it's also affect players a lot, it's very hard to keep expansion stability up to -10 even in Viceroy.
So I think it's necessary to add a new system to prevent it. This is my suggestion.
When a land tile covered by a civ's culture continuously for some turns, the stability level of this tile for this civ will be upgrade for a level. For example, a historical area tile will become core area, a foreign area will become contest area. But tiles that are foreign core area can't upgrade.
This "some turns" could be different for different civs, for the game balance. For example, ancient civs like India, Babylon may need 100 turns or more to upgrade a level(normal speed), America, Brazil and so on may only need 50 turns or even less(normal speed). It's just probable numbers. We could refer to every civ's Culture Rating, Starting Year, the Number of Core Area Tiles and so on, to make this "some turns" more balanced for this game.
My suggestion all above, any discussion's welcomed.
