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?
Both Keith David and Michael Ironside have done a lot of TV as well, so I think it's doable.
I agree that "Eye of the Beholder" didn't give Rolston much to do. Fortunately, he'd have a better gig in the "Enterprise" episode "Canamar" a few years later.
And just a reminder, it's a subtle thing but this is about actors you'd expect
to see in Trek, not ones you want