Now to Mxzs's idea of new city-state types. While I do not like adding new entities needlessly, I do find one thing worthwhile: the mechanics of gradual civ collapse through some of your cities turning city-states.
On the difference between "soft" collapses and "hard" collapses, our ideas are not that far apart. The only difference is one of technical realization. I suggest using a kind of puppet city to realize the soft collapses and saving the city-states for hard collapses. You also wind up suggesting bringing puppet cities into the mix somehow, so I think there may not even be a difference between our suggestions.
The key point would have to do with how to reverse the "soft" collapses.
Me: A new mechanic would let you conquer your own puppet cities, to bring them back under your political control and give you control of their production, coin, resources, etc. An alternate method would involve using a Social Policy that lets you annex them peacefully, as you can do in the game right now.
Elenhil: A new mechanic would let you turn city-states that are diplomatic allies into cities of your own kind. An alternate method would involve simply conquering them again.
I suppose the better solution would be whichever one is easier to mod, since they both rely on finding a new mechanic.
I actually don't think I'm suggesting anything very much when I suggest adding one more "entity"—the "political puppet" entity. It isn't that complicated, and I actually think it could complement your tile ideas very nicely. If you conquer a foreign city and try to take complete control of it—culturally assimilate it—you would set up a battle royale for cultural control of the tiles. I imagine this mechanic could easily feed data into the stability mechanic so that assimilating foreign cities would cause instability, sometimes resulting in revolt and secession by the conquered cities. The solution for civilizations that have to do lots of conquest would be political conquest without cultural conquest—imperial occupation. So, conquered cities could be held as less unstable political puppets, contributing coin, production, and resources, but not culture and science.
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One reason I really like your tile idea is that it might be extended to allow two possibly interesting extensions: Trojan memes and underground civilizations.
An underground civilization is one that does not have political control of any of its own cities, but does have cultural control of at least one. Possibly all of its cities have been conquered by foreign civilizations and turned into political puppets of that aggressor. So, a player might still have cities that are generating science and culture for him. In such cases perhaps he could "cash out" some of his culture for insurgent units or tile control of some of his old cities and their environs. Or, if he is pursuing a cultural victory of some kind (maybe it is one of his UHV conditions, and he is not far from meeting it), he could continue to collect culture points and aim for that kind of victory even though he has been dispossessed of an actual empire.
Or maybe there would be a mechanic that could let him hoard up his cultural points and use them to stage a hostile takeover of the civilization that is occupying some of his land—the Trojan meme. Remember, he would be drawing culture points from many of his old cities, and the civilization that had them under imperial, political control would not. Instead of cashing out his culture in social policies, tile control, or insurgencies, he could save them up and if/when his culture reaches a certain percentage of the total culture generated by one his occupiers he could simply take over that civilization: All his old cities would revert to his full control, and he would get political control of the aggressor civilization's cities. Basically, the alien empire, being a culturally impoverished sort of thing (at least, relative to the culturally rich civilization that it conquered), it is vulnerable to being taken over from the inside by the cultural elites of the nation it conquered. An illustration:
You are Egypt, and you were playing for a UHV victory, but barely missed it. Since you are a cultural powerhouse you generate great gushing gobs of culture from your wonders and your buildings. But your military is weak and your three big cities are overwhelmed by the Arabs. However, they only took them as political puppets, for their production and resources, and the culture generated by Thebes and Memphis and Alexandria continues to go into your pocket. Instead of launching insurgencies, you save it up. Eventually you hit the magic point, and your three cities revert fully to your control. In addition, Mecca, Jerusalem, and the other cities built or taken by the Arabs become your political puppets. In essence, you are now the Arabian Empire, but centered in Thebes rather than in Mecca.
out of it, and probably 
, too, but not 
. And probably resources and no culture and happiness costs, too, to make this option really attractive (though we need to balance not to make it too attractive).
