Let me throw this out, because I've been fiddling with it off and on for some time . . .
First, I like the idea of a 'branching' Tech Tree, in which the basic Technology leads to Branches or Applications. Ideally, there should be about 3 Applications per Basic Technology, and you can simultaneously be researching 1 Tech and 1 Application. That means you will have to specialize on the technologies your Civ actually needs, becasue you cannot possibly research all the Applications. In a way, this takes care of the calls for Civ-specific Tech Trees - you will in effect put together a Civ -Specific Tech Tree in every game, based on the in-game situations.
Second, Civics and Social Policies and 'culture' are also related to technologies and applications. Having certain social policies helps you Apply certain Technologies, and in other cases, may close off (temporarily) some Technologies - your little digital citizens simply aren't interested.
This is an early game example, and assumes a 'Neolithic Start' with a Game Start Date around 10,000 BCE instead of 4000 BCE.
Tech: Agriculture
Resource Requirement:
Food: Korn, Tropical Fruit, Rice
Fibers: Flax, Hemp, Cotton
NOTE: If you have nothing to grow, you will not learn to plant and grow anything.'Korn' is a nice short-hand term for all the similar Grains: wheat, millet, rye, barley, etc
Knowledge Requirement:
None
Allows:
- Unit: Basic Warrior
- District: Settlement
- Building: Granary
- Improvement: Farm
Bonuses:
NOTE: Things that make it easier to Research the Technology by generating a NEED for it.
- Have any Population reach Hungry state for 2 consecutive turns
- Have Water adjacent to exploitable Food Resource tile (marsh, river, oasis)
NOTE: The first shows the Need to plant edible crops, the second to use Fibers for nets and snares to exploit fish and waterfowl, two enormous sources of food for hunter-gatherer types, and frequently used to supplement early agriculture
Applications:
Flood Irrigation
- Provides +1 Food on Floodplains or plains/grassland Tiles next to River or Lake - Provides Bonus ('Eureka') towards Tech: Writing
- Provides Bonus towards Civic: Heirarchy
Selective Plant Breeding
- Reveals Potato, Maize Resources
- Reduces Production Cost of Archer Unit by 20%
- Provides Bonus towards Tech: Agricultural Innovation
Brewing
- Reveals Wine Grapes Resource
- Farmed Korn, Rice, Potato, Maize Resources provide + 1 Amenity (Beer!)
- Provides Bonus towards Application: Sealed Pottery
- Provides Bonus towards Civic: Host Relations
Hopefully, one example shows at least partially what I'm talking about: you cannot easily research even a basic Technology without a Need for it, and while the Basic Technology will give you access to some things, the Applications of that Technology will give you access to more, and some of them may turn out to be Necessities. In the example above, getting access to Potato Resource because you are in hilly country and potato crops can exploit hills much better than 'ordinary' grains (Korn) or Rice.
And there are some sneaky things hidden here (which gamers will no doubt quickly catch on to after their first game(s)). The Sealed Pottery Application simply means your low-temperature Pottery containers (which date back to 17 - 18,000 BCE, so are Starting Tech) can be waterproofed to hold and preserve Food and Drink. BUT it's pretty imperfect, so Sealed Pottery will provide a Need for higher-temperature fired Stoneware which is non-porous, and those higher-temperature kilns will also allow you to produce Fired Brick for better construction and smelt the metals that give you Bronze and, eventually (with Charcoal firing) Wrought Iron.
Oh, and Bronze Tools (an Application of Bronze Working) is a Pre-requisite for The Wheel Tech, which along with Woodforming gives you the (relatively late) Application of Spoked Wheels, which gives you access to Chariot Units, one of the Units whose use provides a Bonus for the Heroic Warrior Civic