My proposal...
Items highlighted in blue are optional to the basic model here, but highly recommended.
First, most techs are divided into several key areas. These are: Seafaring, Agricultural, Industrial, Commercial, Militaristic, Religious, Scientific. Yes, these correspond to the civ3 traits. This is intentional.
Using the civ3 ancient techs as a model (as a *model*, these aren't set in stone), these would be placed as follows:
- Expansionist - the wheel, horseback riding
- Seafaring - map making
- Agricultural - pottery
- Industrial - construction, masonry
- Commercial - currency, code of law, mathematics
- Militaristic - bronze working, iron working, warrior code
- Religious - ceremonial burial, mysticism, polytheism
- Scientific - philosophy, alphabet, writing, republic, literature, monarchy
Seafaring, agricultural, industrial, commercial, and militaristic techs are part of the standard researchable tech tree.
For all techs that are researched as part of the standard tech tree system (all except religious and science techs), Each tech in a given category gives a 5% bonus to any tech research made in the same category.
Religious and scientific techs are NOT researched directly. You have no control over their research. Instead...
- You accumulate one 'meme' per population point in your empire.
- You also accumulate memes for population points in allied nations (1/2 rate), trading partners (1/4 rate) and civs known to exist (1/10 rate). Only known populations count. If you only have information about half a foreign empire, only that half counts.
- Science type city improvements can multiply any memes going towards science techs. Religious improvements can multiply memes going towards religious techs.
- The exact ratio of religion:science memes produced corresponds to the % of income devoted to science research. This models the way that strongly religious societies can gain religious techs but simultaneously tend to stunt scientific research.
- Thus, raising the science rate will boost regular research and science memes, but hurt religion memes. Lowering the science rate will hurt regular research and science memes, but boost religion memes.
- At any given moment, a civ will have 3 techs being researched simultaneously; a player-chosen normal tech, a randomly chosen scientific meme, and a randomly chosen religious meme.
Civs with one of the main tech traits get a 20% bonus to research in that trait. Civs with scientific trait get a 10% across the board bonus (except scientific/religious trait techs). Religious civs get no research bonus. In a game where civ traits have been disabled, this paragraph will of course become irrelevant.
Note that all bonuses listed here are added together; they are not compounded like city improvement bonuses. Having 2 relevant techs plus the relevant civ trait results in all beakers being multiplied by (1 + 0.2 + 0.05 + 0.05 = 1.3), not (1 * 1.2 * 1.05 * 1.05 = 1.323).
Next, while most techs will require the standard 0-2 techs as a prerequisite, there should be additional requirements in some cases. You can't study horse riding if you don't have horses, or how to work iron without a source of iron. This does of course imply that the resources will appear a tech or two before they do in civ3. Also, because some techs will have prereqs from a different tree, research will be more complicated.