The way I understood it, they weren't intending to do another expansion pack unless fans wanted it.
That said, I've got a list of features I'd like to see if they do release a new expansion. (these aren't my ideas, just ideas I like - well a couple are mine but anyway)
Revolutions!
In addition to national happiness, cities have local happiness relating to how much a city agrees with the ideology of the government, religious matters (I'll get to this in a second), and local happiness buildings. Unhappiness from population is part of Local Unhappiness, and in addition certain buildings, policies, ideologies and religious tenants may cause either happiness or unhappiness at a local level.
National Happiness is used for Empire Wide stability, but local happiness can cause a single city, or a few neighbouring cites to brake away from an empire and, depending of the circumstances, join another city, found a new city state or found a minor civ (I'll get to this later too).
If there is high national unhappiness ~20, rebels can spawn near the capital and around particularly unhappy cities. If they Capture the Capital, its a revolution, which may change one or more factors e.g. Ideology, (Policies Piety vs. Rationalism e.g.) or State religion.
State Religion:
The dominant religion in your capital; Bonus Happiness for Cities where your state religion is dominant, unhappiness where it is dominated by another religion.
In addition Religions could have tolerance levels Ranging from very tolerant (of other religions) to very intolerant, decided when the religion was founded (and maybe changed when it was improved?) Basically: More Intolerant religions get a religious pressure bonus and speed bonus and allow the inquisitor unit, but get a greater unhappiness modifier for differing from the state religion or having followers of other religions in a city and have greater Diplomatic bonus/penalties for the same/different religions, while tolerant religions get happiness for having the same religion as the capital, but less/no penalties for differing, and maybe even being able to use the beliefs of more than one religion if the number of followers is high enough, but have less religious pressure and cannot use inquisitors? Also maybe spread slower.
Maybe tolerance is the wrong word there, suggestions for a better one welcome.
Local health
Based of health buildings, local health resources as well as policies, religion, ideologies.
Affects the probability of Plague starting in a city, as well as how long it lasts for in a city. Spread by trade routes?
Minor Civs
Sufficiently advanced City States can become minor civs by building another city. There should probably be a restriction so they can only do it when they are 'advanced'. At least among city states.
Minor Civs become city states by only having one city.
Major civs can form minor civs by local rebellion (as stated earlier).
Losing a capital to a foreign empire should probably cause significant local discontent so large empires start to break up if their capital is taken.
Minor Civs become major civs (i.e. ones that can win) if they become significant enough (e.g. overtake the bottom major civ with score?, build a wonder etc.)
I agree that Espionage needs a bit of work done to it; I would like to be able to recruit more spies, either with money or a lot of productivity (I mean about the same as a national wonder, per spy) as well as more variety of spy missions, cold war style; Encouraging rebel factions who agree with your ideology and/or religion, Poisoning the water supply to increase chance of starting a plague, swaying local and national opinion of my ideologies stealing more that just tech- maps or even great works!
I like the idea of random events returning, and the 3rd Unique was a great idea.
Obviously more civs and city states, also new improvements like the canal (good idea Princeofnigeria) although I think they should be able to cross roads, or at least railroads.