Hello,
It would be useful to have a new constraint type that limits what transformation values the constrained bones can have.
It would have a parent-relative mode and a root-relative mode; it would allow for minimum and maximum values to be set for position (requires separate x/y), rotation, scale, and shear; and each of those would have a mix.
The most obvious application for this I can think of is limb range of motion.
You could, for example, restrain a leg to never bend though a character's body. Take a human character facing right. Setting a range constraint on the thigh bone to parent-relative mode and limiting its angle to something like 350 (angle when thigh is bent back, slightly behind the torso) to 170 (angle where thigh is as far in front of character as possible). And you could give it a 95% mix so instead of stopping, the rotation just slows greatly for 171-349.
More than just simplifying certain animations, this would allow animators to build in a sort of fail-safe to prevent procedural animations from doing anything really weird in corner cases. It could also help with certain pseudo-physics things in spine (constrain a skeleton's translation to a box that acts like walls, ceiling and floor, especially useful on leg IK targets with regards to the floor).
Overall, it could be a major convenience in spine, and simplify certain things for developers manipulating animations at runtime.
Bump bump, tssshhhhh.