All features would need to be toggle-able or JSON controlled, of course.
Reduced visibility, “wet” clothes with cold effect on the player, lightning could “flash” the whole sky (not just some white line on the skybox), howling wind and drawn-out thunder, rumbling. I´m sure you´ve already considered these things, but I feel they should be suggested again. Toxic fog used through the atmospheric volume could be another, more apocalyptic addition.