If I had to pick just one system as a candidate for a modern, rich, smart client, service-oriented GUI, then Joel Bartlett's would be the one, from 1991, that runs on X/Unix in his wonderful Scheme->C system.
Some updating of the programming model would be necessary. The current model is much like NeWS but uses Scheme and X, where there is a single application somewhere using EZDraw as a GUI server.
EZDraw, like SVG, offers a 2+ dimensional interactive drawing environment and so offer the graphical freedom of a web page or PDF with the interactivity of a traditional GUI. I'm hoping that XAML will be just as interactive, since it may become the de facto standard.