Landscape
Its effect on armies
One thing that I would really like to see in this game is a varied landscape mechanic that gives certain units an advantage over others in different places. For example, if you have some rangers and you send them into the forest, they should move much faster, and deal more damage, than for example a large war-wagon which would most likely get stuck. This would give you a reason to create different kinds of units, rather than just spamming "the ultimate unit of *insert cool word*" over and over as soon as you've reached that level of civilization.
Also, lets say that there is a tall mountain-pass that you need to cross. Now, if you bring archers, their accuracy would most likely be reduced due to strong winds and the like. And, if it would be snowing, they would be completely useless since they wouldn't be able to see their enemies.
Civilization IV had a system like this, but it wasn't very advanced. It was mostly like "stand on this hill and gain 20% damage", which isn't very realistic since standing on a hill would also make you more vulnerable to air-attacks and so on.
So I simply think that giving the landscape, and its effect on units, some real thought would give the game a more varied and fun gameplay, where you have to take different advantages and disadvantages of your units into consideration.
What do you think?