his are two things I would like to sugest that could increase the "out of this world" felling and the originality factor of the game
first would be curved space ( proof of concept app
http://geometrygames.org/CurvedSpaces/ ), I belive that with some fine tunnig something like making things after a certain distance bigger and slightly rotated would produce an impressive sight, wickedly distorted space could be really usefull for portals and for the death realm as a whole (picture people walking in the ceiling, a 180° spiral bone bridge or a cliff that isn't really a cliff)
the second would produce so imprecive visuals, and wouldn't be really noticeable, but would give more realism, fisheye vision (like this guy did
http://wouter.fov120.com/gfxengine/fisheyequake/), most engines produce some nasty distortions on greater FOVs, doing like this would produce more organic feel, and increasing the fov as one gets closer to a portalo would also add some nice effect.
