In the new upcoming location which is almost complete ("The Screaming Swamp"), a new terrain system allows extremely large locations (this one is about five miles in diameter, but could potentially be hundreds of miles if you want a really big walking simulator), and also will likely use a new light scattering / fog effect which allows realistic haze and blurring without any noticeable drop in performance, in fact it may boost performance once it replaces the old postprocessing effect that has been used to blur the player's vision when losing consciousness. The new method is faster and can be used for several effects via a single shader.
Screenshot of the fog:
