As hunk92 and Dreyk suggested, a location based climate would probably be a good compromise.
My suggestion (with a couple of years of Arma scripting expierience in the background) would be to make the temperature dependent of the terrain height. Higher locations like mountains would be colder in average, lower regions (e.g. coasts and sea level) warmer. In a later update snowy textures and leafless plants - and maybe even snow fall - could be implemented.
Varying temperatures and winter environments would at this point even make sense as there is already a broad set of clothing, wich would fit well and is currently hardly of any use (e.g. short pants, skirts, T-shirts, winter jackets, winter hunting clothes).
