i have 4 different ideas that would add a little extra fun and challenge to the game. (sorry for the novel)
idea 1: random natural disasters!
earthquakes, large fires, hurricanes, tsunamis, and plagues. they would make a city lose lose population, cause production to grind to a halt, and cause certain buildings in the city to have a higher maintenance cost for a few turns. obviously they would increase unhappiness. you could prevent the effects of natural disasters by constructing things like fire stations, hospitals and coast guard bases in your city. also, if there is a plague in a city and the city is connected to a trade route, it increases the chances of the plague spreading to other cities! oh noes! D:
idea 2: slavery!
woo! a part of history and civilization we want to forget, but an important part. when conquering a city there is an "enslave" choice and you get slave points on the top of the screen, just like other strategic resources, and just like other strategic resources, you can trade them.also, stealing workers with military units turns them into a slave unit. you can put slaves to work on construction projects in your city to speed up the process and produce a "slave" unit that you can designate to a luxury resource production hex such as a mine or plantation and they will increase the output. using slaves, however, increases unhappiness.
later on, there could be a social policy called "emancipation" that gets rid of slaves, turns slave units into workers, adds happiness and starts a golden age. also, a social policy called "civil rights" that adds one happiness for every worker unit you have and starts a golden age.
idea 3: uprisings and rebellions!
A city may start an uprising if for several turns your economy is in the negatives, your nation is unhappy or a city is starving. the first stage is much like an annexed city, it won't do much work and you must either fix the problem or send military units into the city. if you do this, the city may either stop uprising (if you're lucky) and everything will be fine and dandy, or your military unit will be kicked out and the city starts the rebellion stage, where it becomes sort of like a city state thats angry at you. it will start producing military units and you can try to appease them to stop the uprising or you may try to attack them and reconquer them. the appeasing part is much like the quests that the other city states give, such as "disband this many workers/military units" or "make a certain amount of money/happiness for so many turns." there is a way to make the city permanently mad at you and they may try to conquer more cities! that would suck, but its your fault!
ok, lets take a break...go get a snack, take a piss, call your mother...
...ok, are you all back? we're almost done here...
idea 4: airports!
when you discover the flight technology, you must construct an "airfield" in your city in order to produce and house airplanes. later on, you can build a "hangar" which allows for more amount of planes in your city. finally, building a "global airport" will create a trade route to your capital or any other city thats connected to your capital with an airport.
the end! hope you guys at firaxis or 2k like this stuff and add it, thanks