I think the developers can easily port some of the map designs and assets into AOE3 as they now have the same engine.
For example,
Valley of the Kings: a map with lots of treasures (with distinct looks), ancient Egyptian statues and monuments, and mines guarded by outlaw outposts (like the silk road tps)
Variants of Scandinavia, Mediterranean and Acropolis with AOM assets (and even units).
Treasure models
Cheat units
Hero skins