TrekWeb has an exhaustive look at Star Trek: Planet of the Titans, the aborted feature film project from 1976 that was tossed in favor of what became Star Trek: The Motion Picture. The scrapped film from screenwriters Chris Bryant and Allan Scott would have seen Spock discovering Kirk living as a wildman on the former homeworld of the Titans -- a technologically advanced race wiped out by the current residents of the planet, the Cygans. To save the planet from a collision course with a black hole, the Enterprise and its crew enter the hole, only to find themselves traveling thousands of years backward in time. In a twist ending that plays with the paradox of time travel, the crew is revealed to be the Titans themselves, bringing their technology to the distant past of this primitive world.
Producers felt the script was "pretentious" and lacked the elements that made Star Trek unique. That, coupled with drastic management changes at Paramount at the time, also killed the project. Star Trek creator Gene Roddenberry began work on a new Trek television show, Phase II, an idea that snowballed into becoming the 1979 Robert Wise film.
Of particular interest in the article are scans from the out-of-print book The Art of Ralph McQuarrie. McQuarrie, famous for his design work on Star Wars, did a handful of concepts for would-be Planet of the Titans director Philip Kaufman. In those images, you can see a very different looking Starship Enterprise, pairing harsh geometric angles with some of the low-tech design that the ships of Star Wars would become famous for. You can also see McQuarrie's shuttlecraft designs, as well as an impressive-looking asteroid-based space dock. TrekWeb's piece is a fascinating, in-depth look at the Trek that might've been, and it's interesting to see McQuarrie's unique take on the Star Trek universe.