For the OpenGL/3D part of things I'd suggest OGRE. Cross-platform, open source, a large community, and on Windows you get to choose between DirectX and OpenGL, without sacrificing portability to MacOSX and Linux. And the HLSL support is neat. There's also normal mapping, parallax mapping, and a lot of other goodies incorporatied right into OGRE. Think about the possibility of realtime animation rendering at modern game quality... Combined with good animation tools such a package would definitely gain some foothold in the 3D CG world.

I love dockable windows and toolbars in GUIs.
In general, I'm not that enthusiastic about multimodal apps. Switching modes can be a pain. Dockable palettes are usually a more user friendly way.
Then again, some tasks are clearly different. The Setup room for example, that is quite different from "normal" Poser use, so a special Setup node is not unfriendly to the user.
Actually, something like the Setup room would be a suitable plugin candidate.
Materials, however, are part of the standard Poser workflow and should not require a separate mode. Dynamic cloth and hair - and rigid body dynamics? Why not incoporate ODE into this app? - questionable. I use it as part of my standard workflow, but not everyone is that much into dynamic cloth and hair.
Functionality comparable to the face room - I'd say a separate mode. We're talking morph generation here, not a typical part of the standard Poser workflow.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

My gallery   My freestuff