I generally agree with Krusty, but war in Europe was conducted with more clouds than we see today in AH and you very rarely see footage of anything in England when it didn't have some rain. I lived in England for four years and know what the weather is like. If you didn't learn to deal with it in some way, you didn't fight a war.
As I recall reading somewhere, there were times when bases were socked in with clouds and rain for the morning and forecast to be partly cloudy in the afternoon and they would launch those missions.
My priority would be:
- Increase cloud coverage / variability
- Add wind down to the ground layer
- Add light rain / drizzle eye candy
Clouds and wind affect game play in a more strategic fashion as they can impede target visibility and chance to hit target from the ground to service ceiling. That's a very broad impact. Rain tends to be very localized to the lowest band of weather, most often occurring in the ground to 3000 feet range. Adding high and low pressure systems that wander across the map affecting wind speed and direction and clouds along with daytime heating / cooling affecting cloud coverage and air density would be much more interesting to me.
Wind gusts and storms would be pretty low on my priority as they would just be avoided, but if you want to add turbulent conditions below 5000' on sunny days, I'd be for that (more so than rain). I can't imagine trying to fight in thermals under the cloud bases in summer, but I'm sure they did at some point.