I think it would have been cool if Barbara Bain, Martin Landau and/or Greg Morris could have guest starred on TOS particularly given they were filming Mission: Impossible right next door.
All of these pics are great, I wish I had the talent to do some of these myself.
My Desilu crossover fantasy has always been to see the Hogan's Heroes actors in TOS. Imagine Bob Crane as a wisecracking starship captain that always got on Kirk's nerves, Robert Clary and Richard Dawson at navigation and helm, or John Banner as a transporter chief, complete with Schultz accent and mustache. Or Werner Klemperer as a Klingon...