Over in News & Chat, I had proposed that Blender be made specializable for various game-geometry architectures, like Quake-engine brushes and entity attributes.
I'm mentioning this here, since this may require some changes to the Blender code proper.
In particular, Blender would have to support editing of brushes (convex polyhedra) and represent them using a Quake-map-friendly fashion -- a set of intersecting planes.
This need not be supported directly by the code, however; it may be possible to define a plug-in that handles brushes and that uses Blender proper as a user interface, renderer, etc.
Likewise, one could write a plug-in that supports triangle-sided meshes and named animation frames, since these are what the Quake series uses for inhabitant models.
Map compilation could also be handled -- all that is necessary is to call the appropriate equvalents of "qbsp", "light", and "vis" (the Q1 compilation programs) on the map. And the plugin may even be able to tell the game engine to run the compiled map.
These efforts can be extended to other real-time-3D games, such as Unreal-engine ones, Lithtech-engine ones, etc.; I think it overkill for older RT3D games like Doom, Marathon, Duke Nukem 3D, etc., where a floorplan view generally does fine for map editing.
I'm not sure how much I could contribute to such efforts; I'm throwing out these ideas in order to see how useful Blender might be for editing games outside of its code framework.
How about coding it into Crystal Space ? There is already a 3D Doom engine know as jDoom, looking great by the way.