That's a great Idea, but not a new one.

According to the timeline at the above mentioned site, (http://www.campyonly.com/history/timeline.html) Tulio Campagnolo first had that idea in 1924, and patented the mechanism above in 1940. That there my friend is the great grandaddy of derailleurs, and if you want one, you don't gotta invent nothin'. Just go down to the bike shop, and axe 'em for one o' them newfangled gear changy thingies.