Out of all the main cast of TOS, Walter Koenig and William Shatner are the only two who never appeared in another live action series. Trials and Tribble-ations was edited into previous footage.

Bones was in Encounter at Farpoint. Spock in Unification. Scotty in Relics. Even Sulu and Rand were in VOY's Flashbacks. There's still time for Shatner to appear on a live action show, even if briefly. Sad that Walter never got the chance.

Edit to add : Nichelle Nichols didn't either. I wonder if TPTB considered asking her.