Space: 1999, PHASE II, and retro Sci-Fi of the 1970's
I'm sure someone along the way has also thought of this, but although we didn't get a new Star Trek series in the 1970's, we DID get a nifty little Sci-Fi TV show titled SPACE: 1999, considered by many to be the spiritual sequel to STAR TREK. Although not necessarily set on a starship, the series had an intrepid crew of explorers much like STAR TREK, and featured many of the philosophical, thought-provoking types of stories that had made STAR TREK so special. Add to that the similarity between SPACE: 1999 and STAR TREK: THE MOTION PICTURE in the aspect of costumes, sets and props, and you have an idea, however vague, of what PHASE II would have been. Simply put Kirk and crew in the place of the SPACE: 1999 cast, and change the setting from Moonbase Alpha to the Enterprise, and ta-da! You have, in essence, PHASE II.
"To boldly go where no man has gone before"
--ADMIRAL JONATHAN ARCHER--