First of all I'd like to see some basic deficiencies in the interface/gameplay/speed rectified. The game has some great new strategy but it falls short in many areas.
After much searching I found eotinb's thread
here that gives notifications when one of my cities grows ... this makes the amount of time I have to worry about adjusting my citizens much less.
There are also mods by others for better advisors, an area of the game where civ IV has gone backwards from civ 3. The Civilopedia needs to be redone too.
And I would like to see an option like "Show friendly moves" for our OWN units ... so we dont accidentally bump into a lion with a settler for example, and this would also be helpful in times of war and long sea journeys.
Just the basics that we lack, that could make our enjoyment of civ even better. I am a relative newbie to civ - I played older versions and recall them as being much more friendly than this release.
And of course, MUCH faster. The game runs like a three-legged dog on many/most systems as far as I can tell and on mine it's slow enough to detract significantly from the game. And it's not hardware that's the problem. It's the game software itself.
So, what I would like to see in an expansion is just the basic tools and options to play the game easily, as well as an improvement in speed.
And I suspect that for the time being I will have to rely on modders to do what Firaxis should have done in the first place - make civ an awesome game.