Always have wondered what-if...
I'd love to hear Honor Blackman or Joanna Lumley doing the "Space, the final frontier..." intro, mainly because I think they have such great voices.
Saw a 1968 interview of Joanna Lumley the other day, she was so effortlessly charismatic and funny even then.
Honor Blackman is a pretty sterling pick, I think. The kind of woman who got cast to play goddesses and had that ringing-declamation style of acting absolutely down pat.
To go a little more obscure -- he's something of a "that guy" but I've always found him to steal the show in anything he appeared in -- I've often wondered about Maurice Dean Wint (who most people know as the psycho cop in Cube).
Great picks, folks, keep 'em coming!