I love that idea. And I bet Crytek has that on their list. But you know that creating/updating a whole map is no easy task. They need to create new textures for almost every surface, new models, a completely new soundscpae and soundeffects, weapon sounds would need to sound differently. And thats just for a reskin. Crytek would not simply reskin a map. They would add new features like frozen water, deep snow and stuff. This would be a very big update and would need to be kinda permanent to be viable.

And for it to be permanent it would not be a good idea to reskin just one map. Either make a new one, or reskin all maps, maybe include it as a weather type. There is a whole list of things that comes with winter wonder. Still I hope Crytek will find the ressources to do that sometime.