A sprung chain tensioner would allow always having the right amount of tension, without too much friction, and if it was sprung with a constant force spring, it would have the same tension no matter the orientation, and if the chain was pulled taut, it wouldn’t cause any extra friction from the tensioner. Also, constant force torsion springs (of varying strengths, or all in a small strength so that more could be used to get the proper force) would be very useful for counter-springing arms because they have constant force throughout the rotation of the arm, and because they probably wouldn’t need to be replaced nearly as often as rubber bands.
TheRealCedz has created an arm that has constant tension throughout the motion with the use of clever anchors for rubber bands. Here is his reveal:
I believe this year, 1492A and X used them as well, to great effect
This wouldn’t really be that useful, since the force that opposes the arm is not constant throughout its rotation. Maybe on a winch and pulley lift or something like that…