That is a great idea. I hadn't thought of a way to involve the Hollow World really. That would be one way to do that. I'll make a note to add that in later. What era do you think that should be in? Speaking of which, I need to list the common techs for the mod so you guys can help me set up the tree.
Here's a list of the techs and what they do:
1st era:
1) Brewing
Improvements: Brewery
Resources: Wines
2) Bridge Building
Enables Bridges
3) Code of Laws
Improvements: City Watch, Gallows
Citizens: Watchman
Governments: Monarchy, Republic
4) Diplomacy
Enables Diplomats, Rite of Passage Agreements
5) Domestication
Resources: Livestock, Horses, Elephants, Camels, Wolves
6) Early Prospection
Resources: Gems, Gold, Silver
7) Edible Crops
Resources: Grain, Sugar, Rice, Cocoa, Corn
8) Farming
Worker Actions: Irrigate
9) Fishing
Improvements: Fishery
Resources: Fish
10) Mathematics
Units: Torsian Arm
11) Perspective
Improvements: Temple
12) Ranged Combat
Units: Early Ranged Units
13) Riding
Units: Early Mounted Units
14) Road Building
Worker Actions: Build Roads
15) Survival
Units: Scouts
16) The Wheel
Units: Mobile Siege Craft, Chariots
17) Tool Making
Units: Improved Worker
18) Trade
Improvements: Market
Enables Trade Embargo
19) Warrior Code
Units: Spearmen
Improvements: Barracks
20) Weaving
Resources: Silks, Cotton
21) Wood Construction
Units: Small Boats
Improvements: Harbor, Lumber Mill, Wind Mill, Timber Palisade
Resources: Quality Wood
22) Writing
N/A