As a former military infantary men I'm verey awere that the terrain and the light conditions dircetly influence on the way we perform or militay operations. That's why I tought about add rivers (not all of them, just the main ones like Nilo, Amazonas, Missisipi etc) to the game. A river is a natural barrier against enemies, he can slow the advance of trops or even stop it.
How it work:
1- The river won't be like the water we already have in the game, it will be just a water course with more places that you can use to cross it. These points are the places where troops can go trough, as the exist in real life;
2- The time to cross it can be about 1h30 and units with higer rank can cross it faster;
3- Only the infantary will be able to do it for obivius reasons, but that open a chance for us to add a new troop to the game: enginners!
4- Enginners: a suport unit able to bulid bridges ans some type of fortification to defend agaist infantary or armored units,it also can bulid bridges; these bridges can be uptdated so heavy vehicles can use it (like heavy tanks). Also the enginners can be uptdated in order to bulid bridges and fortifications faster. Of course these bulidings operations will cost some resources, but that can be included in the daily cost of the enginner to make it an expensive and in some way "special" unit;
5- The reason for adding rivers in the game is to make it more realistic but still fun; it will open a new type of combat and strategy that has to be develop to use this new type of terrain. It will also make the combat harder (as it is) and require more strategy of the commanders.
The other suggestion is add the day/night cicle, the reason for this is very simple: Is something that affects directly the way we conduct military operations. At day time is easyer to spot targets from the air for example, at night its almost impossible without some night vision googles. So I think that this resource will also add the strategy and make it more realistic.
How it work:
1- 12h of day light and 12h of night;
2- The field of view of terrestrial units will be decreased by 40% and only commandos wont be affected at night;
3- Air units will have their field of view decreased by 50% at night and also cause less damage;
4 - Naval damage against other naval units will be decreased (cause naval combat is harder when you have no light);
5 - Commandos will gain some 20% of additional damage at night ;
6 - Units in defense will gain some 15% of strength at night;
In resume: It will be harder to attack or spot enemies at night.
Sorry for any mistakes I made, english isn't my first language, but i'll try to answer all the comments!