What actors would you liked to have seen show up on the original series?

Frank Sinatra
Dean Martin
Peter Lorre
Marlon Brando
Steve Mcqueen

Orson Welles, Dustin Hoffman, William Holden, and Marlon Brando

Boris Karloff

Sammy Davis Jr.

Paul Scofield
Jack Webb
Sidney Poitier

David Hasselhoff was still too young but he would have been amazing in a gritty and angst-filled role

I'm trying to come up with a few more that haven't been mentioned yet in this thread. The ones who were around then....
George Hamilton
Bobby Darin
Anne Bancroft
George Segal
Anne Baxter
The last two appeared together on The Alfred Hitchcock Hour.

I agree with James Mason and I'll add Claude Raines. He was in a few episodes of Alfred Hitchcock Presents and I always enjoyed him in them.