R.J. Burke might be available. You mean all those guys out there in need of work can't wear makeup? Dancing goalposts, dude.
OK. That doesn't change the fact that Star Trek has reused actors multiple times in several different roles, some with no make up, and some within a part of each other. Mark Lenard, James Cromwell, Michael Dorn, David Warner (in back to back films), Majel Barrett, Jeffery Combs (playing 3 different characters on the same show), Suzie Plakson, and Robert Duncan McNeil, who played a character so similar there is still a debate as to whether or not Locarno was just an alias for Tom Paris. Here's a full list. Not seeing the issue.
Discovery could afford both Jason Isaacs and Michelle Yeoh, though, whom I would say are much higher demand actors than Weller at this point. As others have pointed out, Weller's resume is largely TV over the last decade and change. Honestly, apart from Robocop and Buckaroo Banzai (both from the 80s), I can't really name any of his other rolls off the top of my head.
Why is that? Alexander Marcus exists in the Prime Universe too. He doesn't just look like Peter Weller in the Kelvinverse.
While there's good story potential in the character of Marcus, I think the established tendency of DSC to recast roles even when the 'Kelvin Timeline' actors are available (Sarek, Amanda, Spock, Pike) suggests that it's unlikely that they will use the same actor (whether for legal, merchanising or other reasons... ¯\_(ツ)_/¯
You don't have to go back to the 60s to see actors being reused: see David Warner, Suzie Plakson, James Cromwell, etc. Heck, Warner appeared as two different characters in two different Trek movies, back to back, and squeezed in two episodes of TNG around the same time. And, in any event, Weller's appearance on ENTERPRISE was thirteen years ago. I doubt if it would confuse anyone. Plus, I have to ask: if audiences in the 1960s could cope with actors being recycled without their heads exploding, what makes us think modern audiences are somehow less capable of coping with such things?
Interesting, and a good option. But instead of or in addition to that, can we get an appearance by the character I momentarily thought you were referring to? http://memory-beta.wikia.com/wiki/R.J._Blaise
Did the Prime Carol Marcus appear out of nowhere? Some sort of parthenogenesis? What's her midichlorian count?
Yes, it would. @CorporalClegg, do you mean the actor who played astronaut Bill Anders (From the Earth to the Moon)... ....and Capt. Ed Tucker on SVU? FWIW, I think he would do great on Trek but I want to see him back on SVU so Tucker and Benson can get married.
Honestly, it depends. Sometimes, sure, as with James Cromwell, they're all but unrecognizable under the makeup, but let's be honest here: most of the time we all know it's the same actor and we just roll with it. And it certainly doesn't mar our enjoyment of the individual movies or episodes. Look at David Warner again. How many viewers didn't recognize him when he appeared, playing different parts, in the movies and TNG? We all knew it was Warner, regardless if he was playing a human or a Klingon or a Cardassian. And has anyone ever insisted that, say, "Chain of Command" would have been a better episode if they had hired somebody new to play the Cardassian torturer? I don't think so. It's all about suspension of disbelief. Anybody who watches enough movies and TV shows gets used to the idea of seeing the same actors playing different characters. That doesn't change just because some of those characters exist in the same "universe." In our heart of hearts, we understand the difference between the actors and the characters, and that we're watching a theatrical performance, not reality. It's not that confusing or distracting.
And sometimes we even see the same actor portray multiple characters in the same episode. Jeffrey Combs, for example. I'm fairly sure there's at least one DS9 ep where he plays both Brunt and Weyoun.
Bottom line: Whether you're talking movies, TV, or live theater, there's a social compact between the audience and the show: that for the duration of the performance you will accept that the guy you are watching is "Admiral Marcus" and not Peter Weller, even though, of course, you may recognize Weller from other performances and other roles. The audience goes along with the illusion for the sake of the experience. That's how acting has worked for thousands of years. STAR TREK doesn't require a different set of rules.
I don't think Discovery will be officially Prime until Vaughn Armstrong gets a part in an episode. (he can play Marcus )