Hi sunrisebacon,
thanks for your feature request. I appreciate that.
I think the decision of what's a natural humanoid movement is more an artistic choice. A cartoonish character might be able to perform motions that a real-life human would never be able to do. Or if the humanoid character is a robot, it could perform very unnatural movements that still make sense in that context. I've even seen videos where UMotion was used to create a "breaking bones" animation.
You can use the humanoid muscle tools (shown in the Scene View when no bone is selected) to apply modifications to the bones that are within a range that one would typically consider as natural.
Best regards,
Peter