@SgtSlick:
I can appreciate the sentiment but to be honest, C2C is already set up this way to just about the right degree at this time.
What it means if you take it much further is that many buildings simply go unbuilt ever, you never get the feeling of completion, you never feel like its time to start working at building for war, production is by far the ultimate yield, and you end up feeling like half the buildings in the game don't have any purpose to be there because they simply won't get built.
It becomes as much choosing what not to build as it does choosing what to build. And if it were to become much more like this then war would never be an option because you'd always be stretching to get your cities into a satisfied state.
However, this whole scenario is also to some extent a positive thing because it means you've got some challenges in how you arrange your build strategies so to some extent its one of those things where you make it harder and its more fun until you suddenly hit a point where its no longer enjoyable and while it gradually got fun as you made it harder, it drops off drastically when you take it that last inch too far.
IMO, from what little experience I have in playing with current settings, in the prehistoric age at least, it currently strikes the perfect balance.
I suppose the captives mod changes probably make a very big impact there that I haven't experienced enough of to see. I suspect that costs should probably now be modified up exponentially around the time they start becoming a real game factor.
@MagnusIlluminus: The concept of establishing some positive feedback on the specialized yields and commerces of cities that have ended up being highly specialized is not a bad one. More positive feedback in general is the only concern that shows there. So if it were a subtle effect, as you suggest, then its probably a good idea. I also have some concepts for GP pts that will be inspiring to specializing cities.
@Joseph: Anytime you take a more specialized city style approach you're also going to be compelled to take sacrifices in other areas to do so in those cities (not building things you'd normally have standard everywhere else) so the idea would be to enable both strategies to be equally effective. I don't think anyone is looking at eliminating strategies and their value in suggestions like these, rather I believe its more that the game would benefit if we can find ways to introduce new value into new strategies, enabling more diverse playstyles to emerge. This is my goal when modding anyhow
@AIAndy: Well... cool! You get it and have pushed the concept forward a bit! I like

We can certainly keep things somewhat simple by keeping the buildings private or public definition rounded into building categories, but each building could interact with the property you're discussing differently.
Any given Publicly built buildings would also be more or less likely to be built over other options based on an equation of demand from the citizens (they would want to build what they feel is most needed for them) and that equation could be rather complex.
I also like the idea of destruction of privatized buildings in a privatized manner too - businesses do go under all the time!