I agree, I think that weather would be a GREAT addition.
And anyone who says framerates will suffer should look into upgrading from Windows 95. I don't say this to be snide, I say this because AcesHigh has COMPUTER requirements that were top of the line around 5 yrs ago, and even economy systems (<$500) nowadays exceed them. In the end, its more important to have a good internet connection anyway.
As stated, wind would have almost no impact on framerate, and dynamic wind/weather would be nice.
Rain can be added as a 2D (or nearly 2D) effect, much like FlightSim98 did with almost no performance hit.
Already, I've noticed better clouds, and they REALLY are making a difference in gameplay. You fly into them now instead of through them.
PS. I agree, some snow/other terrain would be fun to have on some of the older maps.
Remember, small changes go a long way.