SR, I know what you mean.

Weather should be a major factor in battle, as it was in Shogun and Medieval.
In Medieval I'd never fight a battle with my english longbows in the rain - never.
While in Rome I knew I was going to win anyhow, so I never really bothered with sometihng as ''unimportant'' as the weather.

Battlefields should get muddy. horses ( Knights ) should recieve charge penalties as should archers,units should move a little bit slower, your line of sight should be constricted in heavy storms, archers should not be able to fire fire-arrows when it rains etc etc.