What if you were able to mute one part of the motion clip. For instance, you mute the legs part of the motion clip, so when you blend the motion clip with another motion clip (exp arms) it would give you the ability to combine the motion clips. I attached an image to make it clearer.
This would be great. Currently it's only possible to mask animations with the mask feature in Motion Puppet, but there are only a few basic animations available.