Using fossil fuels for energy is the most important thing that has happened to human civilization since the development of agriculture. The internal combustion engine has changed everything from the way we grow our food to the way we fight oue wars. Modern civilization would completely collapse without electricity, largely generated by coal and natural gas. And let's not forget about all the negative impacts of fossil fuels, including economic crashes associated with oil prices and climate change from carbon emissions. Yet the entire Civilization series has done a poor job simulating any of this. If you don't have any coal, the worst that happens is you can't build railroads or Ironclads. Farms are only marginally more productive in the modern era than they are in the middle ages. If Civilization 6 is ever made, I propose making coal, oil, and natural gas non-renewable strategic resources. Each turn, each city with electricity and each motorized unit will consume 1 unit of the appropriate fossil fuel. Over time, resources will peak and peter out, forcing players to acquire new sources by force, exploration, or trade.