What it might do that would be useful (to me) is improving diplomacy and adding more content and layered systems that would add more versatility and depth to the game.
What I don't understand: people are assuming a hypothetical expansion to add diversity, options and whatnotever.
On the other hand, many people claim that Civ5 was designed for the casual player who would be attracted by the current shallowness.
So, people are assuming that Firaxis alienated their old fans by design decisions to acquire other, new audiences. Once this has been achieved, these new audiences shall be alienated by adding more options?
Somehow, this doesn't fit.
The ones speaking in favour of Civ5 have mostly claimed to not like either religion, corporations, espionage, health, civics, whatever. In short: 90% of the ones in favour strongly disgust any variety or additional options.
Now, if these are the ones Firaxis wants to attract, what would you expect from an expansion?
Most probably there will be some additional DLC, some new civs and "natural wonders" (Garden of Eden, Atlantis, Yggdrasil and the like) and maybe the option to communicate with your Facebook "friends".