I would definitely like to see a very in-depth technology tree. I presume that some technologies may give a faction a combat advantage over an another faction, some may increase the life expectancy of the faction members or make their survival a little bit easier (such as Herbalism, Cooked Food, Leather Clothes, etc., as mentioned below), some may influence the way they expand, and who knows what else.
Here are some "ancient/classical era" technologies I thought of:
Plant Gathering -> Herbalism
Stone Tools -> Improved Stone Tools
-> Stoneworking -> Masonry -> Stone Houses -> Castles
Simple Woodworking -> Wooden Clubs -> Hunting Spears -> Bows
-> Hunting -> Trapping -> Leather Clothes -> Leather (Houses) Tents
-> Woodworking -> Wooden Houses
Fire -> Cooked Food
Early Mythology -> Burials -> Paleolithic Art -> Stone Sculptures & Figurines
-> Cave Painting -> Proto-writing
-> Philosophy
-> Religion -> Primitive Religious Architecture
The Wheel -> Cart-making
Pottery -> Primitive Food Preservation
Agriculture -> Irrigation
Dog Domestication -> Domestication of Farm Animals -> Horse Domestication -> Horseback Riding
Mining -> Early Metalworking -> Currency -> Copper Tools & Weapons -> Bronze Tools & Weapons -> Bronze Armor -> Iron Tools & Weapons -> Iron Armor
Fabric -> Boat Making -> Sailing
#NOTE: Techs such as "Copper Tools" could be broken down into "Copper Axe", "Copper Sickle", you name it.
When it comes to techs which are tied to a specific material (e.g., Metalworking, Copper Tools, etc.), a faction should, in my opinion, be able to research them ONLY if it has access to the resource. For example, a mountainous, landlocked tribe would hardly ever think of Sailing.
Some techs, such as Religion or Philosophy, could later be expanded into many smaller techs - theories or ideas/ideologies, which could influence the society of a certain faction. These theories could cause unrest, start a revolution, a civil war or the creation of fanatics. The possibilities are virtually limitless (as long as you are able to code it in, of course :P).
There could be many ways of acquiring new technologies, for example:
-at random - "heureka" moments (RNG-based?)
-through trade (chance to acquire technologies through trade routes)
-through diplomacy (an agreement about sharing technologies - agreements may be possible only after a certain technology, for example)
-through war (acquisition of the faction's supplies and territory)