Currently only the whole panel content can be tranparent with compositor. What I'd like to see is being able to set different transparency setting (including 'Make active opaque') for panel (i.e. the gray background] and its content (i.e. dock icons, launchers, etc..)

My goal is to have almost fully transparent panel with less transparent icons where only icons and not the panel itself get opaque on mouse-over.

A really cool but completely unimportant bonus feature would be a fade transition effect when a transparent part of a panel gets opaque on mouse-over :)

I'm not into any fancy effects here, I think this could, with enhanced size settings (see my other enhancement request), improve usability.