Colonial expansion by Europeans into the East would be my choice for an expansion pack. Instead of Indian tribes, you'd be making alliances with East Indians, Zulus, and Chinese.

I also like the idea of a new commodity, spice, which can only be grown on some terrain and only sold on other terrain --placing a plantation on a spice patch allows a plantation to produce it, you decide when to convert the spice into a defenseless cart for transport, and you're likewise limited where on the map you can place the dropsite building that can convert spice to food/coin/XP. Personally, I like the idea of having a max of one "Spice Market" on the map that can only be built where there are at least 12 manors within a radius, but it might make more sense to just have the spice drop site be your starting HC once you enter Age 3.

Some problems on carrying spice over water --are the carts carried inside ships, or do they morph into a small cutter when they reach water.

But I like the idea of a very long distance to the drop site.