Roddy McDowall definitely fits Trelane like a glove. But could be pay Rojan or Koloth??
Going back to the original post candidates, Keith David and Michael Ironside were mentioned. I'm not sure whether the TREK casting agents could afford them, but both excel at movie roles.
But sometimes sci-fi TV may not serve certain character actors,
depending on the show. Remember Mark Rolston from ALIENS? He's another Ironside-type, but when he appeared on TNG he seemed incredibly drained of personality. Or was it because he was playing one of Picard's low-ranking crew?