Funnilly, the weather in the first act changes when you move the camera across the river before the Blighted Village and over the river infront of temple of Selune. The skies darken, rain starts, there is even some sounds of rolling thunder. Even the clouds change and show an actual storm in the distance over the hills, all dark sky and silver small clouds infront of it. If you push the camera far enough and then turn sideways you can see the clouds changed into dark cover over the area with goblins. But if you move characters in that direction and so the camera moves with you the weather immediately returns to blasted sunlight and those clouds disappear.
That atleast shows they can and did implement different weather. Its actually in the game, only its not currently allowed to last, or show itself in full.
The Underdark does show they implemented the actual darkness in this game about coughSharcough... and i really love the fact that not having the dark sight actually makes you almost completely blind in the dark. So its not just fake dark like most games and movies use. At least in Whispering depths, the Underdark has its own glowing thingies that make it less actually pitch black dark.
So... on the ahem, bright side, these features are already in the game. Created. Implemented. Functional.
From there it isnt so impossible or somehow difficult or "against some design preference" to expand the weather and the dark to show up and cover more of maps. Especially because the good side, Selune is mistress of the stars and the moon. Cant get those while the bloody sun is blasting.