I would love to be able to alter transparency of rectangle boxes. This way, I could show clients things like lightboxes, etc.
Transparency is now available for all stencils that have a configurable color option like background, text or border. Enjoy!
Dave W. commented
I agree that this would be a great feature. Just like Brian mentioned, especially for lightboxes.
I'd be very happy if it were just for rectangle boxes, but however is easiest to implement, go for it. If its easier to add it to all elements, I don't think that'd be a terrible thing. But most uses I can think of would be for rectangle boxes. Possibly images as well.
Yes, transparency would be great. But not to all object, just for background.