Evidence Spock Had Feelings For Michael (And Vice-Versa)
Did Spock develop feelings for Michael that go beyond mere empathy? Based on what we learned from Michael's behavior during Star Trek: Discovery's "Brother" and what we know about Spock and Michael growing up on Vulcan together, we can make some guesses. Spock has always guarded his personal life but we do know he was betrothed to marry T'Pring as a child, a marriage he did not want, which was ultimately annulled years later in The Original Series episode "Amok Time." Star Trek never elaborated on why Spock didn't want to marry T'Pring; it was inferred it was because he wanted to dedicate his life to Starfleet. But will we learn in Discovery that it was actually because, during those years on Vulcan, his romantic feelings were reserved for Michael?
From Michael's point of view, it's actually logical she could have been romantically involved with Spock. As a teenager living on Vulcan, she would have found extremely limited options for a boyfriend since she is human. The Vulcans' general disdain for humans would have left her isolated and lonely; meanwhile, we know Spock also endured the same type of prejudice, since he's half-human/half-Vulcan and had struggled to reconcile this. Naturally, they would have found kindred spirits in each other and things might have blossomed from there, especially during their tumultuous teenage years.
Spock should have also undergone Pon Farr, the Vulcan mating ritual, for the first time when he hit puberty (like he did on the Genesis planet in Star Trek III). The question is, was Michael present for this? When Michael told Sarek that Spock accepted her "for a time," the implication is that he accepted her as more than just as his sister. Michael's reluctance to provide any further details to Sarek fuels this line of speculation. Whatever did happen between Michael and Spock, Michael takes the blame for the relationship going sour, indicating she was the one who rejected Spock or ended what was going on. It's even possible Michael broke Spock's heart.
What This Means For Star Trek: Discovery Season 2
A possible past relationship between Spock and Michael in Star Trek: Discovery would echo the relationship in the J.J. Abrams-produced Star Trek films between Zachary Quinto's Spock and Zoe Saldana's Uhura. After all, it's now canon that Spock is attracted to beautiful and intelligent African-American women. Meanwhile, even though she didn't realize Ash Tyler was really the surgically-altered Klingon Voq, Michael's season 1 love affair still counts as her having the capacity to fall for an alien.
By virtue of being a prequel series, Star Trek: Discovery looks to be capitalizing on this prime opportunity to answer a lot of questions about Spock's past. When fans meet him in The Original Series, he is the cool, efficient, fully-formed Vulcan we know and the best friend of Captain Kirk. Star Trek: Discovery, meanwhile, gets to explore the younger, rawer Spock that fans have only gotten a glimpse of in The Original Series' pilot, "The Cage." What Star Trek: Discovery reveals about Spock and his past with Michael will then add extra nuance to many of the choices and actions we know he'll make in the future.
As Star Trek: Discovery season 2 progresses (and as Discovery continues to inch towards Star Trek: The Original Series), Michael is bound to follow the clues in the star map Spock left behind on the Enterprise, which should lead her not just to Spock but likely to the Red Angel as well. However, just as fascinating will be Spock's reunion with Michael and the revelation of just what happened between them during their teenage years. The truth could well be something entirely different, but if not, it would be perfectly logical if Spock and Michael shared a romance in the past. The friction between them in the present will no doubt color how they embark on saving the galaxy together on Star Trek: Discovery.