I am looking for alternate contemporary actors that you think would have made the original series even better.
I’ll start with James Coburn as James Tiberius Kirk. I think he would have interacted with the crew even better than Shatner ever did, and the show might have lasted at least two years longer.
You?
Peter Falk as McCoy?
Martin Landau as Spock.
LLoyd Bridges would have made a decent captain, based on his performance in that Battlestar Galactica: TOS two-parter “The Living Legend”.
Danny DeVito as Scotty?
Diana Rigg as Edith Keeler in the City on the Edge of Forever.
He was, what, twenty-one with no acting credits? I’m not sure that’s good a fit for a high-ranking guy with a thick Scottish accent as, oh, say, Sean Connery…
Kirk – John Saxon
Spock – Raizô Ichikawa
McCoy – John Hoyt (better than D. Kelly, imo)
Uhuru – Tina Turner (Pam Grier was just a little too young)
Sulu – Bruce Lee
Chekov - ???
Scotty – Elsa Lancester (radical, I know)
Yeoman Rand – Jayne Mansfield (after the 1966-67 season, Joi Lansing)
Trelane – Andy Warhol
So, then, Leonard Nimoy is available to play Rollin Hand?
Nice choice, but note that this would lead to endless nitpicking, speculation, and fanwanking about his missing eye. (Then again, the same thing happened with James Doohan and his missing finger.)
How about Burt Ward as Chekov?
bruce would of been great especially since in the books sulu is supposed to know martial arts … (and he could of helped the fight scenes look better)
Seam Connery would have been good.
How about Davy Jones?*
“Get me a Davy Jones type”
“I can get you Davy Jones…”
“uh…PERFECT!”
“Stand up, Mr. Chekov.”
“I im standeeng up, Keptin.”
Chekov would give Kirk too much competition for the space babes. Chekov would look at them, and little stars would appear in his eyes, and it would be all over.
.
*aren’t you a little short for a starship trooper? 
I see your point in casting, but they were both pretty old to play Rand.
David McCallum for Chekov.
TCMF-2L
“Oh, one more thing, Jim…”
Kirk – Alan Hale, Jr
Spock – Russell Johnson
McCoy – Jim Backus
Uhura – Dawn Wells
Orion Slave Girl - Tina Louise
Spock’s Mom - Natalie Schafer
Red Shirt #17 - Bob Denver
Klingon Crew - Harlem Globetrotters
Kirk: Elmer J Fudd (be wery qwiet - I’m hunting Kwingons)
Spock: Bugs (It’s Romulan Season)
McCoy: Daffy (It’s Klingon Season)
Uhura: The Road Runner (Meep)
Scotty: Porky Pig ( the di-di-di-dilyth…ah, no power Captain!)
Every Red Shirt: Wile E Coyote
Klingon Kommander: Kosemite Kam
“Ahead warp factor three, little buddy.”