Ideas for resources
I had a few new ideas for resources, not sure if they are possible but....
1) different sizes for resources, for example an Iron resource could be good for 50 turns or for 200 turns before it is exhausted. The icon could reflect that by being larger or smaller. Maybe a different tech or research could extend the life of a resource, such as better farming techniques could improve the yield of a Wheat resource.
2) more varied resources, some mods have Lead and Potatoes for example.
3) underwater resources such as manganese or coal (near the coast)
4) maybe prospecting units could be used to search/work a hex to improve the chances of a resource showing up. This way, even if you build a city where no resources are near, there is the possibility you would have a better chance to discover some.
5) mines could be at different depths/difficulties. Some Coal could be right on the surface and would take a worker 2 turns to build a mine, some Gold could be harder to reach and would take 8 turns for a mine.
6) resources outside cultural boundries could be mined/farmed like colonies were used in Civ3
Anyway, those are a few off the top of my head. Got any to add?
Resource rules and ideas
The only addition I would add here if possible would be resource amounts. If you have one resource you can make an unlimited number of units that need that resource. Perhaps with only one horse resource, you can only support x amount of cavalry. Or you could limit how many cities can produce units of a type requiring a specific resource. One Iron source, only x number of cities can produce cannon or knights or medieval infantry. This would add an important element to game play. Most foreign policy is connected to procurement of strategic resources. This is why imperialism and colonies were neccessary.
One of the most crucial resources, Civ 3 tracks overall population, a rule that determines the actual amount of available population for military service and limiting the number of units allowed at certain population levels. You would have available manpower and used manpower in the resource screen that would allow you to plan your military.
Building of units, weapons etc.
If as in many other systems, each city had different things they could produce at the same time.
For instance, regular building of improvements and wonders, than a barracks for ground units, Shipyard for ships, aircraft factory for aircraft, automotive factory for tanks, apcs and artillery. Perhaps make the number of factories you can have limited by the number of resources you have, and make them expensive to maintain. For example have only one horse resource, that will support only three cavalry depots, have only one bronze source, it will support only three bronze age unit barracks. To make it easier to code allow only one military production facility per city. That city might be producing city improvements at the same time as producing medieval infantry in a steel age barracks.
Do you see the wheat supply in the West disappearing any soon? What about the rice in China? The wine in France? These things have been around for thousands of years and probably won't disappear soon. While resource limits may make sense for some resources like iron or horses, for others it's just arbitrary and will destroy a lot of the sense of development in the game.