Uh... First off, I can't really see how adding trees would do anything aside from make it more difficult to ski and possibly detract from the realism (contrary to popular belief). You see, when you add trees, yes, they can act as a scale, by which to measure things, but everything else is like the ground or a metal tower. When you shoot them, nothing happens. I common sense dictates that a legionairre's rocket+tree=no more tree. Problem is, destructible items are hard to create in such a way as to make them realistic.
Next on the list is mud. We don't even have water yet, let alone other viscious liquids. Water is something I'd like to see one day [but not yet].
Then comes rain. This might pose an issue for GPU's, idk. It would depend on the way it would be coded. If it was just drawn directly in front of the camera, it would be less realistic, but it would also require far less. From there, an increase in fog ought to do the trick for long range. Rain could be cool, but once again, there are more pressing issues.
Of course the idea of rock towers and whatnot is fine, but my major complaint with this idea is plantlife. there are vines, trees, and various other green things that wouldn't work well IMO in a game like Legions.
