Weather Effects: Rain. RAIN RAIN RAIN!!
I think that it is about time that we have some the map(s) with some real rain. Like the wet stuff that falls out of the sky and lands on your face as you bleed out... Yeah, that stuff.
It would be even cooler if we could have the maps change weather and foliage according to the time of the year (spring, summer, fall, winter etc), but I think that it would be good enough to have some kind of weather that occurs year-round in the southern USA. (Rain).