Lucas originally wrote Star Wars as one huge story. It was sliced and diced and finally pared down to the first Star Wars movie which was Episode 4. No one knew if it was going to do well or not, so originally they just tried to make a movie out of the most "Gee Whiz" part of the story. When it was a hit, they revisited the original story for the sequel.
The second sequel, or the third in the original trilogy was written quite differently from what eventually made it to the screen. It was supposed to have a "darker" ending and there were no Ewoks. The primitive people on the forest moon of Endor were to be Wookies, but Lucas thought that no one would believe that Chewie came from such a backward place.
The latest buzz that I have heard is that when the prequal triology is complete Lucas is looking into selling the franchise so that someone can finish the final trilogy (Episodes Seven, Eight, and Nine). They supposedly deal with the New Republic facing the Rise of the New Empire. In total speculation-land, it has been put forward that the main characters in the final trilogy will be Han and Leia's children - one on the dark side of the Force, one on the light. It will end with a final showdown between the siblings, yada yada yada...
I kind of have the feeling that we have already seen the best "story elements" and that the rest of the saga, unless substantially changed from Lucas's original vision, will be rather predictable.
Bring on The Twin Towers!
That makes sense because George Lucas is kind of past his prime right now. Usually it takes three years for him to complete one of the movies. So at this rate, the last episode would be completed in 2014, there's no way Lucas will still be producing movies by then!