Currently, GTK+ provides at least two different drawing models - the one for widgets and the one for cell renderers. Anyone using a canvas widget also needs to deal with the canvas model (or, in some cases, models). A single model for "something needs to be drawn in the given region, so please do it", presumably centered on Cairo, would unify all these different approaches, and allow several things to be done that are currently either impossible or only feasible with hacks. The most obvious example is putting widgets into a TreeView, but it really goes significantly beyond that.