Add the ability to individually control the weight of each axis (in free edit mode aswell).
This would allow you to do things like invert one axis while keeping the other two axis following normally.
EDIT: I previously said it was possible to do with two constraints. However, this isn't true. Inverting one axis specifically using two rotation constraints (one locks x, y, and the inverted constraint one locks z) followed by checking "solve in local space" leads to unexpected behavior.