Hmm.. maybe someone could add it with a patch?
Buildings could be impassible for all except flying mecha--unless the building is rubble, in which case it could be walked over but not rolled over.
I'm not too excited about the snowy tundra idea, as GearHead is supposed to be post-apoc (which are generally in hot climates--not to say there wouldn't be any snow). However, I too think that more mountainous terrain would be cool. I wish the mountains in GH were broader and sweeping, like a real mountain range, instead of little hillocks here and there. Also, it would be neat if the terrain transitioned more gradually between grass and desert, with maybe some swamps and wastes in between.