Since there's currently only clear or rainy weather, I think it would be nice if new weather types were introduced someday
. Here's a couple of thoughts I've had on weather:
Day/Night: a ±5 swing is too harsh. With a swing like that, night races pulverise day races and vice versa. I think ±3 would be safer.
Fog: -3/-4 combat for missile units? Decrease LOS for most units
Hurricane: -3 combat for fliers and -5 for missile units?
Rain: same as Day/Night; ±3
Sandstorm: natural HP regeneration disabled and most units buffeted for 1 damage every few seconds? Possibly also a combat debuff for missile units too? Decrease LOS for most units
Snowfall/Blizzard: Units get -3 Speed (except units which can cope with the weather like many High Elves, Frost Dragons etc)
Volcanic Ash: Decrease LOS for most units, natural Hp regeneration disabled and most units take 1 damage every couple of secondsMagical Weather
(weather usually brought about by spells)
Meteorstorm: Causes meteors to fall randomly from the sky all over the map.