Around the same time Ralph McQuarrie's concept art was helping make Star Wars a reality, he was designing a new Enterprise for a Star Trek movie that didn't get off the ground. Check out his radically different, Imperial Destroyer-esque starship.
Reading through the articles at Memory Alpha and Forgotten Trek, it sounds like Star Trek: Planet of the Titans went through a variety of different script drafts, some of which were pretty off the wall and none of which would have been that great. In one version, the Enterprise crew would have been sent back in time a few thousand years, to become the legendary Titans that they were sent to find. In another version, Kirk would have gone missing, presumed dead — but actually living on a primitive planet as a wild man — while a discredited Spock would have gone back to Vulcan to purge his human half.
Director Philip Kaufman, who also took a crack at writing one of the script drafts, explains what his version would have focused on:
My version was really built around Leonard Nimoy as Spock and Toshiro Mifune as his Klingon nemesis... My idea was to make it less "cult-ish", and more of an adult movie, dealing with sexuality and wonders rather than oddness; a big science fiction movie, filled with all kinds of questions, particularly about the nature of Spock's [duality]-exploring his humanity and what humanness was. To have Spock and Mifune's character tripping out in outer space. I'm sure the fans would have been upset, but I felt it could really open up a new type of science fiction.
Spock and a Klingon played by Toshiro Mifune "tripping out" in space? It would have been memorable, at least. In any event, the Planet of the Titans movie was shelved in favor of the Star Trek: Phase Two television series — which in turn was converted into Star Trek: The Motion Picture. But during the time that Planet of the Titans was in the pipeline, McQuarrie did some designs for a new Enterprise, along with production designer Ken Adam, who'd worked on some James Bond films. These designs were never used, although they wound up being seen in the background in Star Trek III and one Star Trek: The Next Generation episode. And McQuarrie went on to work on Star Trek IV: The Voyage Home.
Here are some images of McQuarrie and Adam's reimagined Enterprise, including some images that show the saucer section separating from the main hull. [via The Mehallo Blog]
Separated saucer section of the Enterprise, image by Ralph McQuarrie
More images of the saucer section
A shuttlecraft design by Ralph McQuarrie, among other things
Enterprise interior by Ralph McQuarrie