There are a number of actors who tend to work a lot in sci-fi / fantasy productions, or characters actors whose "type", one would expect to lend themselves to various roles on Star Trek. For whatever reason, though, it just never happen. I'm just thinking of actors one would expect to be a natural fit in the show, but never did: Michael Ironside: Besides having one of the most bad-ass surnames out there, his resume's got a number of sci-fi titles (V, Total Recall, Starship Troopers, Terminator Salvation, Lois Lane's father on "Smallville") and, much like the late Charles Napier, he's long been one of the go-to guys to play unfriendly tough guys/military, etc. I could've definitely seen him as a Starfleet admiral involved in something shady, like Pressman, Dougherty, or Leyton. (Try picturing him doing lines from "The Pegasus" or "Insurrection". It's pretty interesting). Keith David: To me, he'll always be the voice of Goliath from "Gargoyles". ("I've been denied everything, even my revenge!!!") I always thought that powerful, booming voice of his would make him a great Klingon or Jem'Hadar. (And as we know, there were TONS of Star Trek / Gargoyles actors overlapping, so what's one more?) Who knows? Maybe he couldn't handle the prosthetics. A lot of actors can't. Robert Urich: We had a "Quantum Leap" reunion in the first season of "Enterprise" when Dean Stockwell showed up, but, for whatever reason, DS9 never gave us a similar "Spencer" reunion. Perhaps as Cal Hudson in "The Maquis" or some other guest character with a connection to Sisko? Xander Berkeley: Another actor typecast as jerks and badguys, though I think he deserved an Emmy nom for his portrayal of the terminally ill George Mason in season 2 of "24". Star Trek probably would've typecast as a heavy-of-the-week as well, but it would've been fun to see an episode on his resume.