This topic has already been tackled a couple of times in the past, but Damn I love imaginary retro-casting...Here are a couple of related threads and below my personal choices for 60s TOS casting (I still like' em):
http://www.trekbbs.com/showthread.php?t=149386
http://www.trekbbs.com/showthread.php?t=226171
Re: What Famous Guest Star Should Have Been On ST, But Wasn't?
Checking what was on TV during that time period, it would have been awesome to have as guest-stars:
1) Kurt Russell (at 11, he'd co-starred with Elvis in 1962, doing the odd TV guest spot until Disney featured him in 1969)
2) Dennis hopper (he was persona non grata in Hollywood in the early 60's, but had a small part in Cool Hand Luke in '67)
3) David Hedison of Voyage to the bottom of the Sea (was on that show until '68, but he'd've been cool)
4) Bill Bixby or Ray Walston (but both were on CBS for My Favorite Martian)
5) Ed Asner (he appeared on Mission Impossible, but not ST)
6) Harry dean stanton (also did Cool Hand Luke at that time)
7) Lloyd Bochner (awesome character actor)
8) Telly Savalas (did The Dirty dozen in '67, but was doing TV guest-starring roles just before, notably Combat)
9) Burgess Meredith (joining Frank Gorshin and Julie Newmar, but he was the best actor of all the Bat-villains)
10) Lee Majors (doing the Big valley for ABC, but if Shatner could guest-star on Valley, why not Lee? can you imagine the cockfight between those two?

)
11) Ron Howard (was finishing up the Andy Griffith Show at the time. He was about 13, I guess he'd have been on the awful Melvin Belli episode)
12) Guy Williams (was on Lost in space for CBS, but I'd have loved my favorite Zorro to be on, especially since Guy had so little to do the second season onward)
13) Leslie nielsen (still in serious mode, but had just finished Peyton Place and other guest spots)
14) James B. Sikking (he did TV back then, so why not 20 years before ST 3 ?)
and certainly NOT least
15)Diana Rigg (unfortunately doing TV's Avengers until 1968, just a year or two before she did Bond), ooooh, that catsuit!!
