Ethan Peck may not be returning as Spock in the third season of Star Trek: Discovery, but Spock still has a role to play in Discovery's adventures. Star Trek: Discovery Season Three sees Burnham and the Discovery crew entering a new Star Trek era, the 32nd century. Spock is long gone by then. While Star Trek fans know about Spock's long career in Starfleet and eventual disappearance into and death in the Kelvin Timeline, Burnham knows nothing of it. Showrunner Alex Kurtzman says that Burnham will investigate her brother's fate and that what she discovers will impact how Discovery's third season plays out.
"I'm trying to answer your question without spoiling too much," Kurtzman says after a question from SFX Magazine about how the past weighs Discovery's characters. "I'll give you an example. It's safe to say that in the spirit of what I just said, I would look to the past and I would try to understand what happened to the people I love. So Burnham is absolutely going to look to the past and try and understand what happened to Spock.
"We all know what happened to Spock, but everything that we now take for granted, she has no understanding of. So, she will dive into that and she will get to see what became of her brother. And that will actually impact the story for her in a significant way. I've just told you more than I told anybody about that!"
When Burnham left Spock 930 years ago, she'd helped him work through his emotional struggles. He emerged aboard the USS Enterprise bridge appearing much closer to Spock of Star Trek: The Original Series than he had in prior episodes of Discovery. His immediate story continues in the Star Trek: Discovery spinoff Star Trek: Strange New Worlds. Fans know Spock becomes Capt. James T. Kirk's first officer before obtaining the captain's rank himself. He later becomes a Federation ambassador and then disappears while trying to stop the supernova that destroyed Romulus.
That's where his story ends in the Prime Timeline. He emerges in the Kelvin timeline, meets his younger self, helps resettle the Vulcan people after the destruction of their homeworld, and later dies.
How learning about Spock's fate will affect Burnham and Discovery's future remains unknown. Fans will have to tune into Star Trek: Discovery, streaming new episodes Thursdays on CBS All Access, to find out.