I think it looks really great, particularly the mud. My only suggestions would relate to the sky (I really do hate Bryce's skies; combo Brycean sky and volumetrics is my favorite). Also the portal's field might look better if you used either a terrain or Wings to make ripples and then put a simple reflective material on it (lol like Stargate). It just looks like a force field to me right now. Alternatively, you could try setting up a simple scene and using that (heavily modified, of course) as the portal to show what's behind it. Like the really, really cool portals in Serious Sam, if you've ever played that game. Those are my thoughts. =)