Make it so sometimes it will rain. That does nothing, but it is still cool and adds really nice atmosphere to the world. Snow would be nice too...
Lightnings with rainstorms... Day and night system...
Another idea is ''scary night'' - everywhere will be ghosts, that will move too fast (a bit faster than the speed of person that moves on road), you cant kill them, they do nothing, but their eyes are shining and bring panic to your soul.
In different weather there is different music. In normal day and night there is no music. In rain there is really sad melody. In rainstorm - the same melody as in rain, but with lightning sound.
In snow there is happy melody, maybe ''Jingle bells''.
In the ''scary night'' there is REALLY SCARY MUSIC. Something classic, maybe ''Dies Irae'' (really scary).
Please add it if you want Atmosphere to be epic =)