Some ideas.
If you think any of them are worthwhile, you can try them for version 1.0.
1. Coastal Defense for Ships.
All ships currently get 10% defense when in a coastal tile.
I am not sure what this is intended to represent.
In any case, it seems that from a game standpoint it would make more sense to give any small advantage to the defending ships in their home coastal waters (defined as coastal squares within their cultural boundary.)
This could represent familiarity with local conditions, abstracting support of shore batteries, lack of fatigue of the crew, etc.
2. Workers Can Work
It would be good if there were something useful to do with workers after railroads are built.
I suggest that workers moved to cities can be ordered to "work".
One coin per turn as long as they "work".
Allowed with Guilds.
Becomes two coins per turn if they "work" in a city with a factory.
When you want, you can move them out and do something else with them.
Not intended as a major use of workers, but can come in handy from time to time when you have nothing really useful to do with them.
One could limit the numbers working in a single city somehow, for example, some fraction times the city size.
3. Shrine of the Fertility Goddess.
An idea for a new wonder.
(Don't know whether the name is the best.)
Gives +1 food for every farm.
Can be built with Mysticism.
Becomes obsolete with Biology or sometime before.
Maybe it is faster to build if you have access to corn, wheat, or rice,
or can only be built with access to grain.
Requires a Granary in a city before the shrine could be built.
Cost in hammers should be high, perhaps a little less than the Great Lighthouse.
4. Deep Sea Fishing Fleet.
Proposed new building:
It would represent the ability to send out a deep sea fishing fleet from the coastal city in which it was built. No actual ship would be moved, but presumably the fleet is going outside cultural boundaries and bringing back fish or whales.
(The reason I did not make it a unit is to avoid the player having to move the fleet.)
Can only be built in coastal cities that already have a lighthouse and harbor.
Cost somewhat more to build than a harbor.
Technology required is Optics or perhaps Astronomy.
Initially + 1 food, + 1 coin, + 1 hammer.
This increases to + 2 food, + 2 coin, + 2 hammer, upon getting steam power or another similar tech.
This increases to + 3 food, + 3 coin, + 3 hammer, upon getting refrigeration.
5. Customizing the Speed of Science
Allow an option to adjust the science speed when you customize the game, similar to the other current player options at the start of the game.
(The speed change is for all players, including AI, compared to what it would otherwise be.)
The speed at which science advances depends on many factors: single player difficulty level, multiplayer, map, style of play, etc.
In some games science gets well ahead of historical.
For example Tanks could be invented in the 1700s rather than the 1910s.
Some players may find the average rate of science advancement in the games they typically play is not ideal for their enjoyment.
There could be several speeds such as: normal, slower, slowest, faster, fastest.
It could be programmed by multiplying the cost for researching technologies by an appropriate factor.
I suspect it would work best if the factors varied by era, with no adjustment for the first era, a modest adjustment for the next era, a bigger adjustment for the next era, etc.
It might also be necessary when you decrease (increase) the science speed to increase (decrease) somewhat the benefit for knowing other civilizations who already have the given tech. (I assume this reduction in research costs is still a feature.)
6. Customizing the Cost of Military Units
Allow an option to adjust the cost of military (land, sea, and air) when you customize the game, similar to the other current player options at the start of the game.
(The change is for all players, including AI, compared to what it would otherwise be.)
There could be several levels such as: normal, higher, highest, lower, lowest.
It could be programmed by multiplying the cost for building military units by an appropriate factor.
I suspect it would work best if the factors varied by era, with no adjustment for the first era, a modest adjustment for the next era, a bigger adjustment for the next era, etc.
(If military units are relatively more expensive, then there should be somewhat fewer of them built. If military units are relatively less expensive, then there should be somewhat more of them built.)
7. Wonder Cost Dependent on Map Type
The relative value of a particular Wonder depends to some extent on the type of map.
For example, a Wonder that effects every city is more valuable the larger the map, all else being equal.
(The average number of cities per civilization depends on how crowded the map is. The fewer the opponents, the lower the sea level, and the larger the map, the larger the average number of cities per civilization.)
For example, a Wonder that effects every city on a continent is more valuable in a Pangea map than an Archipelago map.
For example, a Wonder that effects every coastal city is less valuable in a Pangea map than an Archipelago map.
I suggest that the cost in hammers of Wonders depend to some extent on the map type.
(This particular suggestion, even if doable, would require alot of work.)
8. Towns and Railroads.
There is currently no interaction between towns and railroads.
Historically the growth of a town was dependent on being on the railroad.
Possible alternative changes:
(1) Growth of villages is doubled when on a railroad.
(2) Extra hammer for towns for Universal Suffrage requires that the town be on a railroad.
One would at the same time, increase the current normal number of turns from village to town somewhat, if deemed appropriate.
9. Some possible new Techs
High Speed Rail
Horse Collar
Stirrup
Clockmaking (Chronometer)
Glass Blowing
Telegraph
Green Revolution (+2 food to farms with corn, wheat, rice.)
10. Radio Effects Maintenance
With the discovery of radio (or telegraph) it is easier to manage a large empire.
Maintenance cost due to distance to palace reduced by a small amount.
11. Tweaking Leader Traits.
Some leader traits may be less powerful than others (on average.)
One way to make such an underpowered trait a little stronger would be to have a free super specialist in the capital.
The super specialist would appear upon the researching of an appropriate technology and would be of a type that makes sense for the trait.
For example, the Aggressive Trait might get a Super General settled in its capital upon researching Feudalism.
It would take some game testing to determine when the super specialist should appear in order to maintain game balance.
A really underpowered trait could get two super specialist, one for each of two different techs.
12. Tweaking Civilizations
Some civilizations may be less powerful than others (on average.)
For example, America gets no unique building or unit until very far into the game.
One way to make such a civilization a little stronger would be to have a free super specialist in the capital after an appropriate tech is researched.
For example, America could get a super specialist when Representation is researched.
13. Ironclads
Ironclads are underpowered in Civ IV compared to Frigates and Ships of the Line.
The designer seems to have had in mind the very first ironclad and the most advanced Frigate.
Even then, historically an Ironclad was easily able to take on 3 Frigates.
(Setting aside the speed difference which is already reflected in the game.)
This is not even close to being reflected in the relative strengths of Frigates and Ironclads in the game.
This could be corrected by giving Ironclads a significant bonus against all wooden ships, such as 75%.
14. Making Health More Important
Currently health is less important than happiness.
Each one unhappiness means a tile not worked or one less specialist in a city.
This is worth let us say for the sake of discussion 3 food.
Each one unhealthiness means minus one food.
Yet until modern times, cities were very unhealthy places to live; the way cities had a net growth was through immigration from the country side.
Revise the effect of health to make the effects more pronounced:
-1 health = -1 food
-2 health = -3 food
-3 health = -6 food
-4 health = -10 food
-5 health = -15 food.
etc.
Effect = (Unhealth)(1 + unhealth)/2.
15. Domination victory should be made "Easier".
On a huge map, where I play, I think 1/3 of land area and 1/3 of population, together with at least twice your closest competitor in each of land area and population, should be enough for a domination victory. This is plenty hard to achieve, but does not take forever.
16. Military Units Away from Home
Define home for this purpose as within cultural borders or within the borders of another nation with which you have open borders.
Each turn a military unit is not home it takes damage.
(The amount of damage should depend on game speed.
Perhaps on marathon, about 3 hit points out of 100 per turn would work.
This would have to be game tested.)
Land units and ships would have to rest and repair from time to time, even if they did not participate in combat.