Good show, guys.
Conceptually, the
Mr. Zero story was ahead of its time--creating a series based around that kind of "outsider" fantasy character only previously explored on anthology series such as
The Twilight Zone &
The Outer Limits. The ideas for Zero's mission had many possibilities for a weekly series because the hero would not be grounded (chained) to any one series framework that would limit the range of his adventures, unlike a police show character who would not--realistically--find himself involved with foreign spies, aliens, etc.
That potential was evident in the
Star Trek episode (
"Assignment: Earth") oft-referred to in TTB #172; I always loved the Gary Seven story because he was not only a human raised by aliens to help mankind out of situations beyond their control, but he was advanced enough to pose a serious challenge to a crew from the 23rd century. He kept Kirk and Spock guessing, along with their own worries about possibly altering history. Despite NBC passing on
"Assignment: Earth", it was one of the most fascinating backdoor pilots i've ever watched, and it was a shame it did not go to series--or expanded into (at least) a TV movie (similar to what Irwin Allen did with
City Beneath the Sea when ABC declined to pick it up as a series).
Roddenberry was fortunate to work with Art Wallace who proved his fantasy credentials as one of the most important creators behind
Dark Shadows. Certainly no fantasy slouch.
High C, if you're really interested in well-researched
Star Trek history, you should check out
https://www.facttrek.com/; the people behind it have unearthed numerous facts about the series, while debunking many of the myth or straight-up lies pushed by hacks like Cushman. That site is such a joy for TOS fans.