Yeah, Phun is cool... something like that at runtime would be excellent.
The ability to create objects and load them as objects in some sort of custom library would be sweet.
Then you could upload/send created physics objects to others for use in games or whatever. Anything from a simple rope, to something more complex like a suspension bridge... whatever you like.
So yes, a more improved design-time physics creation... as well as being able to save objects separately to the cap.
If I had to choose between one or the other, I would say better design-time creation/manipulation.