Well, I wasn't planning on posting the list but since you are throwing ideas out there, Kitten, here is the list that I have come up with along with what they do/allow/require.
Added Techs: new 48 (20 Changes to Existing Techs)
Ancient Era:
Farming Starting Tech Makes visible Wheat, Rice, and Vegetables Starting tech for Expansionistic Civs
Domestication requires the wheel allows horseback riding Can use livestock resources
Slavery allows Slave Market, Requires Mysticism
Elephant Training Allows Elephant Rider and Elephant Archer Requires Polytheism and Domestication
Boat Building Allows Mapmaking Requires Pottery and Bronze Working - Allows Galley and Trireme
Drama Allows Theater Requires Literature and Philosophy
Tactics Requires Iron Working and Writing - Allows Sun Tzu's Art of War
Weaving Requires Domestication Allows access to silks
Aristocracy Requires Code of Laws required for Monarchy
Seafaring Requires Mapmaking Allows Longboat
Change to Pottery Requires Farming
Change to Mapmaking Requires Boat Building
Change to Horseback Riding Now requires Domestication
Change to Democracy Rename Egalitarianism Requires Philosophy and Code of Laws
Change to Republic Rename Republicanism Requires Currency and Code of Laws
Change to Monarchy Rename Divine Right Requires Aristocracy and Polytheism
Middle Ages:
Agriculture Literature and Horseback Riding Allows Irrigation Requires Feudalism
Stirrup Starting Middle Ages Tech Required for Chivalry Allows Knight
Clockworks allows improvement town clock and Big Ben Wonder Requires Invention
Explosives Requires Chemistry
Strategy Allows Military Academy Requires Gunpowder and Education
Leadership Requires Strategy and Stirrup Allows Dragoons
Jurisprudence Requires Education
Guilds Requires Feudalism
Mercantilism Requires Education
Textiles Requires Invention Allows Textile Mill
Modern Legal System Requires Federal Republic Allows Prison and Supreme Court
Fundamentalism Allows Govt. of Same Name Requires Theology
Social Contract Allows Constitution Requires Printing Press and Jurisprudence
Federalism Allows Govt. Federal Republic Requires Social Contract and Economics
Change to Chivalry Requires Monotheism, Stirrup and Feudalism
Change to Invention Requires Agriculture and Engineering
Change to Education Requires Guilds and Theology
Change to Metallurgy - Now requires both Leadership and Explosives
Change to Magnetism Requires Navigation and Physics
Change to Banking Requires Mercantilism
Industrial Age:
Emancipation Requires Steam Power Allows Emancipation Proclamation
Legalized Gambling Requires Radio Allows Casino
Pharmaceuticals Requires Scientific Method Allows Drug Store
Motion Pictures Allows Movie Palace and Hollywood Requires Electronics
Mass Media Allows TV Tower Requires Radio and Corporation
Civil Liberties Requires Radio and Emancipation Allows Civil Rights Movement
Refrigeration Requires Electricity and Corporation Allows Supermarkets
Guerrilla Warfare Requires Espionage Allows Partisan
Unionization Requires Marxism Allows Labor Union
Automobile Requires Mass Production, Required for Mobile Warfare Allows Troop Transport
Socialism Allows Govt. Social Democracy Requires Marxism and Civil Liberties
Totalitarianism Requires Nationalism Allows Communism with Marxism, Fascism with Corporation Allows Police Station
Marxism Requires Industrialization; allows Communism with Totalitarianism Allows Union Workhouse
Fascism Allows Govt. Fascist State Requires Totalitarianism and Corporation
Change to Communism Allows Govt. Communist Regime requires Marxism and Totalitarianism
Change to Automated Transportation Renamed Mobile Warfare Requires Automobile
Change to Steam Engine Doubles Work Rate, Allows Laborer
Change to Radio Doubles Work Rate, Allows Engineer and Radar Station
Change to Medicine Allows Hospitals
Change to Sanitation Allows Sewage Treatment Facility
Modern Era:
Advanced Tactics Requires Computers and Synthetic Fibers Allows Special Forces
Game Theory John Nashs Governing Dynamics - doubles output of all economic buildings Requires Fission
Machine Tools Beginning Modern Tech Required for Computers and Roller Coasters
Roller Coaster Requires Machine Tools
Plastics Requires Rocketry Required for Recycling, Synthetic Fibers, Space Flight Allows HumVee
Pesticides Requires Ecology Allows Modern Farm
Data Encryption Requires Computers required for E-Commerce
E-Commerce Requires Data Encryption and Game Theory Allows Global E-Bank, increases commercial buildings
Change to Recycling Requires Plastics and Ecology
Change to Space Flight Requires Plastics
Change to Synthetic Fibers Requires Plastics
Refridgeration - allows more cattle resources on map as refridgeration means they can be transported further to be provided to wider markets.
How would you go about doing this? Would you add additional resources?
Black Death - upon invention of this tech make granaries obsolete for a while (till the tech after) and this way slow population growth. I am thinking myself how to implement this particular era of history in Europe/Asia.
Why would anyone want to discover it? I guess you would have to make it a starting tech and a requirement for anything after it.
Dark Ages: a tech that takes AGES to research and brings no benefits. Aim to re-create that period of history after the classical period where technological change slowed down considerably.
Since the longest any tech can take to discover is 40 turns, I think you would have to have multiple Dark Ages techs in order to really simulate the effect.
Fertilisers (off-shoot of chemistry and industrialisation -if possibe): reveals more grain/wheat-esq resource (I kinda have implemented this already!)
I think I will rename my Pesticides Tech to Fertilisers. Thanks for the idea.