Another thing that MIGHT be a nice addition, although I'm not entirely sure about the balancing, would be a forestry, a building that instead of planting crops, re-planted trees for harvesting material, possibly at a slow-ish rate.
I'm not sure if it was an intentional decision, but the "enemies" are actually in some ways more "good" than the player, they are protecting all the trees and nature from complete and utter destruction by humans, when I won the game earlier, there were almost no trees left, I had transformed the landscape into a desolate wasteland, and the forces of nature (the enemies) failed to stop me.
Slightly poetic, sort of a "Humans Are the Real Monsters" type situation.
http://tvtropes.org/pmwiki/pmwiki.php/Main/HumansAreTheRealMonsters