A realistic system where certain terrains will hinder foreign movements and fight abilities. For example, the crusader states invading saracen lands will be slower due to the heat and wont be as effective in battle causing them to be not as effective in a fight, any soldiers in heavier armor will suffer even more. Saracens won't be hindered at all in their home territory as their lighter, unique armor will protect them from the heat there for not bothering them at all. Another example is invading the snowy colder areas where lightly armored soldiers suffer penalties but those in heavier armor will be well insulated against the cold.

I hope the devs consider adding a system like this.