There is also a set of upgraded textures for clouds now, and the 'CFO weather' addon allows multi-layer clouds, winter or fall textures and more detailed weather and wind conditions (but no rain though...)
Sure about the possibility to add anything in CFS1; you can add any scenery you want and extend the game with no limits, which allows building what video game companies usually won't think is worth, like small countries and their airforces, and simulate the invasion of Albania by Italians for example, or combats between English and French Vichy forces in the middle-east, well the kind of small theaters of operation that are usually discarded, but of interest for guys like me who like WWII history in details. This is one of the reasons why I still fly CFS1, but being a 3D modeller on 3dsmax, I must admit I'm rather tempted by building aircraft for CFS3. By the way, is Gmax still available? guess I must have a version of it somewhere in one of my HDs, but not even sure; and where can I get export plugins for CFS3?