the thing i'm more interested in is the interactions between the cities.. sure building a big ass city in SC4 was cool and all but it got boring pretty fast. and while sure you could buy/sell electricity, or pay to have a region take your garbage, there was no real negative effect for the regions around that city.. you could exploit the system way to much since that region was basically paused while you were in another region. so as long as you never went back to that region after building it up you were perfectly fine.
well if you play single player, and build a city and never go back to it, that city is tech saved in a point in time and will never change. but things like pollution can come from that city you never visit over to one you are currently playing down wind.
now this is where multi player plays a roll since unless the other player is offline that other city wouldnt be sleeping and would be progressing along side your city with power/water/resource needs changing as their and your needs change.
so single player you could still cheat the system by setting up power/fuel/water citys and then just never visit them again but if you were to play a true multi player game where everyone is building what they want and where their needs could change or they could choose to stop selling you power or water is where it gets interesting.