Two constraints that I think would greatly benefit UMotion are a Driven constraint (very similar to Custom) and a Limit constraint.
Driven - The same as Custom but you can link the value to a value of the Driver. For example, link localPosition.x (with an offset so the 0 value can be set) of the Driver to localRotation.x of a bone. This would allow for building complex rigs with simple to use controls the same way Animators do in Maya/Blender. For example, this could be set up on each hand with one Driver per finger (and thumb). Moving it closer/farther from the hand opens/closes each finger using multiple Driven Constraints each with their own scaling/offset values. Another example is for wrist twist bones, the localRotation.X of the hand could be linked to the localRotation.x of the twist bone, when scaled to 50% the twist bone will automatically rotate with the hand at a reduced amount to give a good wrist rotation. This could be chained together for more complex setups with multiple twist bones.
Limit - A limit constraint would limit the position, rotation, or scale of a Transform to be within a certain range. For example, bones could be limited on a character to prevent unrealistic joint rotations without needing to manually fix them. Used with a Driven constraint, the Driver could be limited to only move on 1 axis within a certain range so it doesn't cause issues with the Driven.
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