First I appologize for my bad English. I am not a native speaker.
I would like to suggest some things I thought would be benefitial for this game. I have done a 30min search on forums and in wiki to make sure I dont suggest anything that is already working or has been suggested. I hope I managed this. If not, I am sorry for the repetition. Also I am not familiar with the balancing of stuff so I will only talk about ideas and will not suggest any numbers for any effects I am proposing.
Idea number 1:
I have noticed farms production changes depending on the season. To add more variations to the game and also to the trading system, I propose electrical power would also change with seasons. The amount of infrastructure the power plant can support would be the same in every season. But upkeep would be different. In winter, the power plants that use resources to function (coal,oil,uranium) would have their resources consumption increased because of the heating. I know electrical heating isnt the main heating system, but considering the game system and not adding anything more than is needed, it is easiest to provide this "winter heating" effect by increasing the resource consumption of the power plants. This would also boost the wind turbines value since they dont need any resources.
There could also be a "summer effect" where people use aircondition and this also increases the power consumption. This effect could also depend on cities infrastructure level (say summer effects apply above 1000infra) cause a city with more infrastructure is probably more developed and has more airconditioned buildings.
Idea number 2:
Cities already have a railway improvement (called subway but the meaning is there). I suggest a national project "Railway system". It would have two benefits. First it would increase the bonus of every subway and second, it would allow more soldiers to be enlisted per day (railway is a big help for military). To balance this out, I suggest this national project would only work if you have control over your airspace. Or it could be a target for airstrikes directly. Not sure here on details, but I think the idea is clear.
Thanks for your opinions :-)
EDIT INFO: All editing is done only to reduce the amount of gramatic errors I made...