As far as I know there are already quite a few general purpose scene graph engines out there so I would propose to really focus very narrowly on the Poser realm. I have not really thought too much about it in detail but I would propose cross plattform Qt for the GUI (with windows still being the main target platform), Python for internal scripting, OpenGL for real time preview and an interface to PovRay for rendering (with support for other render modules to follow). Plugin support could be implemented via a COM like layer (which is actually not windows specific). I think before anything else the program should be able to read and correctly display any poser 4 content. This is the one thing that I find severly lacking in DAZ Studio which has all kinds of fancy features but actually can't even read one of my custom characters without problems.