A copy of the reportedly original pitch for Star Trek made by Gene Roddenberry has been found online. Although some of the things revealed are generally well known (the ship was originally the Yorktown, the captain was Robert April – who was later canonized as the captain of the Enterprise before Pike in the animated series, etc.) you can see that as early as 1964 several of the ideas for stories that were eventually produced.
You can see from reading that the concept of “parallel worlds” – the idea that plenty of planets are not only capable of supporting life but actually develop humanoid life similar to our own – was extremely important, not just for stories – but for production. And the pitch even mentions cost-saving measures such as use of existing backlot locations in relation to parallel world stories, and re-using other sets that may become available after other productions by adapting stories. Also, the “cruiser” (ship), would “rarely land on a planet”, instead the crew would land via what we know as a shuttlecraft – transporters weren’t part of the original design.
Weaponry was quite different – the ship itself only had “lasser beams (sic) for self-protection only”, while the crew weapons were more familiar projectile weapons, but with selectable ammunition which included a hypodermic pellets that would tranquilize, or “stun”, an opponent.
The principal characters had some slight differences. Aside from April (who was pretty much in the same vein as the Pike and Kirk we know), there was the female Number One (who was seen in “The Cage/The Menagerie”); a navigator named José Ortegas (not really filled in the first season, but bits eventually became Pavel Chekov); Doctor Phillip “Bones” Boyce (the character sans nickname was seen in “The Cage/The Menagerie”; the nickname passed on as we all know); Spock is a First Lieutenant and right-hand-man to April with a slightly more satanic look and is “probably half Martian”; and the yeoman named “Colt”, described as very shapely and wishes to relate to the captain at a more personal level – eventually this became Yeoman Rand in the series).
Episodes that were described and later produced, although with different names and probably a lot of alterations over time:
“The Next Cage”: Became the original pilot, “The Cage”
“The Day Charlie Became God”: “Charlie X”, although the accidental nature probably became the second pilot, “Where No Man Has Gone Before”.
“President Capone”: Became “A Piece Of The Action”
“To Skin A Tyrannosaurus”: At some level likely became “Arena”
“The Women”: Became “Mudd’s Women”
“The Perfect World”: Sounds like it developed into “The Return Of The Archons”
“Mr. Socrates”: This seems to have had elements that became several episodes, such as “The Savage Curtain” and a bit of “Plato’s Stepchildren” and “Shore Leave”
“The Man Trap”: It doesn’t appear they story idea got used, but the title was recycled.
“100 A.B”: Became “The Omega Glory” – with the “parallel worlds” being a very important story element.
“The Mirror”: Some elements likely became “Mirror, Mirror”
It’s fascinating to read some of the other story ideas that didn’t make it, and wonder what could have been – or if they’d even be acceptable now.