The ability to assign predefined variables to properties of assets.

i.e. If I have a complicated mock-up and I want to try some different color combinations, I can assign all rectangles that represent cards to one variable, all shapes that represent headers to another, etc. etc. Then testing different color combinations becomes much more efficient.

Currently I have to go through and Individually select each asset and change the properties. Support for variables would significantly reduce design time and effort.