I would add to that: if ever you can get to it in further development, the most realistic would be several states (according to the date and latitude) for each seasons (like people is proposing and like YOU are doing a bit with 2 shades of winter...) One can imagine Autumn with full colored trees and another period of autumn with no more leafs and very brown feeling, etc... also, seasons could be adapted to the latitude. Northern countries have so little leaf trees and mostly evergreens, so seasons look obviously different. Even further, if one wanted to dream a bit, imagine that your snow cover would follow the real weather snow falls... I know from experience from the air that the land looks quite different after a big snow storm (I am not talking about the dull repeating fields, of course, but the roads, the hills, etc..). Frozen rivers or not? and, as was discussed earlier in the thread, based on temperature, snow cover on top of mountains at the dewpoint temperature... that is a dream. X-Plane (that we all love) has been so DULL all these years regarding all those aspects and your plugin must be the door to all that! (one can look at what they did in DCS world 2.5 that just came out, the seasons are sunning!) Anyway...Just some thoughts!
JEFX