that would be interesting. Making night rades with plans harder or making commandos stronger. Also smaller stacks are a bit stronger then larger ones at night.
I always liked the idea of weather. Limiting Arial movements and slowing land units. Idk about naval.
Night day cycle would be bad. Since people live in diffrent time zones unless its only an graphic detail then its ok. But I think an weather system should be fully added with its own Map kinda like the morale map. Weather system would be perfect for this game!
Instead of a day night cycle, I would recommend a weather cycle instead. That would mean at certain times, weather changes and the battlefield as well. In certain weather, some units should receive movement penalty while others have none. I also suggest weathers that can limit visibility or even cause air force to be unusable. It might also be a good idea to add weathers that hamper ocean cruise and other actions. This would actually help create more diversity and make combat more complex as well as interesting.