I provisionally like the idea of resource depletion, but only if it's offset (to an extent) by discovery of new resources. I don't just mean by mines randomly discovering a new mine/quarry resource with a 1 in 10000 chance, but by a great farmer type unit.
However, not the way great farmers work now. Mineral resources aren't plants and animals. You can't go out and choose to discover oil in this particular plot of land. The Orbis modmod for Fall from Heaven had a prospector type unit, which is what I'm thinking about. He could be sent out and randomly discover new resources. However, he had a chance of failing, or digging up an ancient demon or something (like Moria in LoTR). Probably not something for C2C, but he could instead trigger an earthquake or volcano, or cause a chemical spill that produces fallout or damages the land itself (lush>grassland>plains etc). Also, he could only use his ability when the square he was in and all surrounding squares were absent of resources of any sort. This prevented the map being literally covered in resources. I think the whole thing was done in python and utilised FFH's spell system.
If something like this was done, perhaps it could look at all the resources currently on the map, and give a higher probability of discovering those that are rare or absent. And, it almost goes without saying, the AI would have to be able to use it. I WANT my opponents to have tanks and planes and battlecruisers.