I figure a hypothetical question would liven up the place.
What would you make? An O.S.? A widget? What? And be somewhat detailed, "You don't have to paint the Mona Lisa, just don't draw a stick figure".
For me I'd make a visual/virtual O.S.. Your "desktop" is a 3D field you can customize, the screen being the point of view. The amount of customization would only be limited to your imagination and the P.C.'s hardware. Whenever a music player would be playing it would effect the lighting, feeling and overall mood of the 3D plane.
In such like say if it was a sunny field with flowers and beautiful trees, and you play a song of a genre and subtype and by the tempo would define to the O.S. that it was a sad type song and it would get cloudy and darker, start raining, the flowers would close up. Not just a simple change of scenery but it would continue to change if the song itself picked back up. And if there is no input, instead of a screensaver, in music mode the camera/P.O.V. would automatically travel around the 3D plane.
Webpages and programs would appear similar to billboards you pass on the roads, to me it would look better than everything on a little bar that after ten things you can tell what they are. Unless the program was made specifically for the O.S.
I can't really say this is a new idea, but it is one that interests me, I hope to one day see this instead of the usual "box", gui.