Welcome culpy67. Well, I think we can safely say there won't be another AoK Expansion Pack, but ES will announce their new game either on December or the first months of 2004.
If we try to guess what it will be, the safest bet would be AoE3.
As XpLoDed_Samurai said, that's the best place to go if you want to get some "proof" AoE3 will be made, along with the two articles linked there, there is an interview with a Microsoft person, shortly after the purchase of ES, in which he says they were definitely making AoE3. As for when... no one knows.
Co-Author of the Aztec Civilization Outline for AoM.
Peter Jackson and Guillermo del Toro to do The Hobbit!
Chichén Itzá is one of the New 7 Wonders of the World!