Alas, I have no idea, although I have been searching the archives for the answer to this question.
At the beginning of pre-production on season three (March 29, 1968; two months before 'Spectre of the Gun' went before the cameras) Star Trek
had three final draft teleplays on hand that were never filmed. At the time, they were marked as "rewrite needed." They were:
ST-64 - Darlene Hartman - “Shol” - Delivered 10-9-67
ST-67 - Theodore Sturgeon - “The Joy Machine” - Delivered 11-13-67
ST-69 - Norman Spinrad - “He Walked Among Us” - Delivered 7-12-67 (Grey: 8-15-67)
There were also a number of story assignments at that time that were never filmed. Those were:
ST-83 - D.C. Fontana - “Van Voyt’s Robots” - Story assigned 2-21-68
ST-85 - John M. Lucas - “The Godhead” - 1st Outline Del. 3-15-68
ST-89 - Lee Cronin (Gene Coon) - “Japan Triumphant” - Story assigned 3-6-68
ST-90 - Lee Cronin (Gene Coon) - “One Million, B.C." - Story assigned 3-6-68
ST-94 - Theodore Sturgeon - “Shore Leave II” - Story assigned 3-6-68
ST-95 - David Gerrold - “Bem” - 1st Outline Del. 3-14-68
You can read about most of these at the Orion Press Unseen Elements of the Original Series page
There were also a few Roddenberry "springboards" written at the end of season two (Gene Coon may have also had a hand in these; I'll save them for another discussion). Any of these might have been slated as potential episodes 25 & 26; then again, once Star Trek
was scheduled in the Friday, 10pm "death slot," just about everyone saw cancellation as a forgone conclusion.