I'd like to see waterpuddles forming during rain & dissapearing shortly after it stops.
Same effects with snow, and if its foggy (yet to be seen) it could start dripping from trees etc.

Id also like to see some "Heat" effects distorting the field of view, just like when its really hot in the desert.

Just some ideas, and well the option to turn the effects on/off if they get too annoying or too damn laggy.