Bryan Fuller is one of the co-creators of Star Trek: Discovery. Fuller left the project after a series of delays before it debuted on CBS All Access, but many of his ideas are still present in the show. For example, it was Fuller's idea to redesign the look of the Klingons in the series. It was also Fuller's idea to make a trip to the Mirror Universe. Fuller was a guest on an episode of Robert Meyer's YouTube series Robservations. During the discussion, he touched on some of his original plans for Discovery, including his original intent for the Mirror Universe story arc (first picked up by TrekMovie). Here's what he had to say:
"The thing that really fascinated me in sitting down and crafting the story for Discovery was the human condition. I thought that there are elements in the Mirror Universe that we have seen that have sort of boiled to the broadest ends of the spectrum and everything felt really binary. And what I really wanted to do in setting out was looking at the minutiae of simple decisions that have a cascade effect on our lives. So, it’s not about gold lamé sashes and goatees versus no sash and clean-shaven. It is more about we are at forks in the road every moment of our lives and we either go left or right.
:It makes me think of Joe Menosky’s speech in [VOY 'Latent Image'], where The Doctor has a Sophie’s choice, he can only save one life. And he chose Ensign Harry Kim versus this other ensign and it is a split-decision and it causes his entire program to unravel because he can’t handle how his choice was always going to cost a life. It was his Kobayashi Maru.
"So, there was something in the mistakes made by Burnham in 'Battle of the Binary Stars' that had this ripple, but the Mirror Universe was always meant to be an exploration of a small step in a different direction. So, it wasn’t necessarily the Mirror Universe we know from all of the other series. It was something that was closer to our timeline and experience, so you can still recognize the human being and go, 'What did I do? How did that seem like a good decision for me in that moment and how do I continue with my life forward?' And everything was a sort of an extrapolation out on that. So, there were things that I wanted the Mirror Universe to function in a narrative exploration of like 'Oh f***, if I just didn’t do that one thing, everything would be better.' As opposed to, 'I don’t recognize that person, I don’t know who that person is, because they are a diametric opposite of who I am.' So, that is kind of what the goal was"
It seems Fuller wanted to use the Mirror Universe closer to how an actual mirror is used. In other words, it would be a means for the characters to examine themselves rather than to see a bizarro version of their universe. As it aired, Discovery didn't veer too far from that path. There were no instances of characters meeting their other selves, as the ISS Discovery had been lost or destroyed, but it did offer the characters a chance to reexamine the path that they and Starfleet as a whole were walking down.
Work is continuing on the third season of Star Trek: Discovery. It is still expected to debut in 2020.