Collision detection with the polycounts typical in Poser? It is going to be hella resource-monstrous in anything that doesn't use pure OpenGL (or DirectX) for calculating it (and even then you may still have to hunt down some external libs for it... e.g. GLIDE and such). My best advice is to build with an eye to it, but save it for later, after you (at the very least) have a solid beta.

/P