What about adding an Energy-ressource/-Yield? A ressource, that stocks, is bound to the city, were it is from and can anables some buildings as well as it can make some buildings better, like production facilities. I believe, the problem would be to manage the energy to stay inside the city it belongs to, when it comes from wind- or watermills. So maybe it would be better to make it a yield, if thats possible. Maybe like food or hammers: Some buildings and modernizations give energy, some take energy. Some require a certain amount of it to be build. Buildings, that require or use energy could have an energy-importance, so that in case there isn't enough energy, the buildings with the highest importance gets energy and those which lower importance don't get it.
In the long run, the system could be used to build modernizations like power poles that would make cities share their energy. It would also make modernizations like modern windmills, modern watermills, solar cells and such stoff more logically. I don't think there is any food coming out of these windmills out there. it would also make more water modernizations possible, like tidal power plants. And i still don't think there is any food in space. It's more likely, there's energy out there. I don't find the energy system in Civ4 logically. I think this would give more realism to the game. Today, energy is a problem in many places. without it, we couldn't possibly survive, and it is treated like a ressource. Everybody thinks about, how to get more, cheaper, cleaner energy. World wonders like the 3 gorges dam and a future wonder like a big solar array in the Sahara could give a certain amount of energy to every City on the continent.
I've got a lot of ideas. Anywhere else i can tell them to you except in this forum?