It would be good if Civ could adopt the equiv of Valve's HL2 episodes. Buy the basic game, and then they can continue to update and mod the game overthe next few years or so.
With Civ there should be a huge range of possible expansions. And if done correctly, they could release them so they are all optional.
Warlords ,which has some new civs and a new Warlord Unit (whatever that is).
Then maybe Future Civ expansion, that lets you play to 2150 or something. (I think not that we're in 2006, having a 2050 ending year is a bit to early. As by the time you get to the present day, there's not really much time to use any of the stuff that you get. It wouldn't even need to go to very sci-fi. It would be cool to implement some "modern world problems" , like over crowding, lack of fresh water, oil, etc.
Then an expansion featuring more barbarian/rouge states. Give minor civilisations more importance. Make it possible for a small minor civ to control important resources so you have something approaching the middles eastern situation.
I'm sure they could they could also work on the religeon side of things some more. Make it more benifical to go crusading in the middle ages.
Civ's got the possibilty to be really flexible. Just now, it looks like it will be mainly down to "amature" modders to do a lot of the creative work. But I'm sure a lot of people would pay for some big in depth mods.
Other them to DL via the net at a low-ish price.
