I'd prefer an improved ecology model to the addition of any new civ. If you want more civs in densely populated areas, you need a larger world map (like Rhye's civ3 one). That in turn means longer loading times, a lot of reballancing, etc. If you want to do it, feel free to do it. For me new civs don't really add anything gameplay wise, which would justify longer loading times.
On the other hand don't you think that erosion, lack of (drinkable) water, global warming (that is not caused by nukes but CO2, etc) is an important thing that is even less represented in civ4 than in the previous franchise. Pollution clearing was stupid, pollution leading to global climate change wasn't.