The most lasting contribution that the Star Trek animated series gave Star Trek was the concept of the holodeck, first seen in the animated show.
There were plans to feature something like the holodeck in the original series, but there was simply no budget for this idea. The holodeck is one of those things, like emergency saucer separation, that people associate with the next generation, but the original series could do it too.
There’s a lot of discussion on how Star Trek was a team effort by a lot of different writers (notably Gene L. Coon, creator of Starfleet, the Klingons, and the Prime Directive), but the holodeck was purely Gene Roddenberry’s idea. In the series bible from 1966, he wrote that “entertainment in the Star Trek era is three dimensional and like a film, except it happens all around you and is interactive.”
