I was surprised to learn that the holodeck first appeared in a 1974 animated episode (although it was called the "rec room").

Acc to wikipedia, the holodeck concept originated in a 1973 meeting between Rodenberry and holographer George Dolgoff, who "emphasized the importance holography will play in the future and that if Gene Roddenberry wanted to be accurate he must introduce holography into his Star Trek scripts, including the concept of a holographic “room” for the crew to use for amusement, training, and other purposes."

This got me thinking: The holodeck wasn't beyond 1960s fx capabilities. What if Roddenberry had been exposed to the idea of a holodeck almost a decade earlier, and written it into TOS from its beginning? Aside of course from the inevitable holodeck malfuction episodes (instead of the crew beaming down to planets modeled on ancient Rome or 1930s Chicago, they would get trapped in such holodeck settings), how would this have effected the series?

I remember this proto-holodeck on the animated series. And it set the precedent for the overused "malfunctioning holodeck" premise.

I'm glad TOS didn't have it.