The problems are such: Firstly, this will be a mod so you can only play versus other people who have the texture pack (and you can't play on snow maps unless you have the pack)[unless of course it is possible to include custom .big files with maps for the snow].
Secondly, while new ground textures are easy, and falling snow particles are probably also easy, for a proper snow environment, most structures and map props (and maybe even vehicles and infantry) would need to have secondary skins created for the snow environment.
Anyways, there's some talk about it on a few forums including PPM here.
I myself will be creating my own texture pack that will ship with my mod, but that won't present mismatch errors because it is part of the mod they will need.
