This seems pretty simple to me. Maybe someone else already thought of it.
Please excuse this low-res crude mockup. But you'll get the idea.
Metro and Desktop combined together. Taskbar where it should be or put it at the top or left/right whatevery you want. Metro still works the same and can show more apps that slide in from the sides.
Programs and Windows open in the foreground with Metro staying live in the background, or an option could be used to let the Metro background blur slightly to give more focus to what is in the foreground.