Bump because there's been a bunch of new suggestions added, with most recent being my suggestion for new property of joints: spring.
It would be great if we could make "spring" joints that, if given an axis and some range of motion, will be able to rotate and yet be compelled to restore their original rotation. Suppose you want to make a bendable, springy sword blade. You attach two bars with a joint and give it an axis and a range. Your "sword blade" can now bend, but it will not go back to its original shape unless external force is applied to it. It stays bent. It's not springy. But, if a joint had a spring property that would cause it to want to return to its original rotation (how strongly would depend on joint strength) we could make springy swords, vehicle suspensions, and other fancy stuff.
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