All of what you describe can already be done in poser 5 already. Take a basic medium resolution plane of significant enough size. Using displacement and the shaders, you can effectively create any terrain you want to in much the same way as the other terrain editors on the market already do. Effective use of a simple modeller and some proper planning for material zones, and you can get some rpetty cool stuff. Combine that with something like Runtimedna.com's Microcosm set, and you can pretty much create any sort of terrain in P5 natively.

thou and I, my friend, can, in the most flunkey world, make, each of us, one non-flunkey, one hero, if we like: that will be two heroes to begin with. (Carlyle)