From http://www.jeuxvideo.com/news/2006/00018446.htm:

Une extension serait déjà prévue dans les locaux de Creative Assembly, et devrait porter sur le continent américain et les guerres aztèques.
Which roughly means that an expansion is apparently in development at CA at the moment, and will focus on the Americas and the Aztec wars.

I suppose I'm happy, but this does kind of infringe on my Age of Warlords mod. Still, there will be a good deal of stuff I can plunder at least.