Wow. That transcript on Cinetropolis makes for fascinating reading. It really illustrates how Lucas most certainly did NOT have a grand plan in mind. He was making everything up by the seat of his pants--and there is nothing at all wrong with that.
I like what Lucas said here:
One remaining thing that I want to cope with is putting the whole thing on the scale of a western, making the whole thing work as a western – more of a Sergio Leone western. It can go more into the middle of nowhere where these really slimy creatures live. Essentially, space can be boring. And I would like to get much more into the Seven Voyages Of Sinbad [type of thing]. Now we’ve established the space fantasy, we can move it away from that.
We’re getting down now onto earth and can make it in a different way. It’s getting down to something much more like on Mars, so you’re running around in a strange world.
Much more of a Flash Gordon, swords and planets thing going on there.