I think physics would be a lot more useful if you could change all the settings at runtime, such as immovable, no rotation, contact friction etc, although you can already change some of them I can think of uses for the others. (Ice weapons, turn floors slippery, ie change contact friction).
You should also be able to retrieve all these values in the expression editor.
Also it'd be really nice if you could apply more types of constraints, as well as hinge, you could have weld, rope, slider, hydraulic, spring, etc. (look at garry's mod for ideas heh).
It'd be cool if you could name these constraints when you add them, then you could enable, disable and change their parameters by name.