Brief description of the outcomes that you expect from this feature:
Possibly with opacity change it could enable you to select Sketches and 3d Model that is at 100% in opacity. Basically opacity change is a soft hide feature.
What can’t you achieve without this feature?
With opacity set lower, it will help you visualize, instead a hard toggle on and off.
If this is to difficult to implement, a great shortcut would be a way to “Pick Face Color” (incl transparency). Is so, you could pick of present item or even build a small selection on-the-side to use.