Spock's death
In The Princess Bride, we discover the concept of "Mostly Dead", which fits Spock's entire "death" to a tee. In the film, the main hero, Wesley, is thought to be dead. That is until Inigog Montoya and Andre the Giant take Wesley to Billy Crystal and future Star Trek star, Carol Kane for help. There, Crystal reveals that Wesley isn't dead, he's just mostly dead. If you've never seen it, watch it, it's a trip.
That's basically the best way to explain what happened to Spock at the end of Wrath of Khan. He "died" but didn't actually die. He, somehow, placed his soul/conscious/memories/essence into Leonard McCoy before death, essentially saving his "soul" like you'd back up a file on an old floppy disk.
They were able to then download Spock into a newly grown body that thankfully looked just like Spock. It's overly convoluted and stupid. The way they did this scene in Into Darkness makes so much more sense, I digress though.
What would have happened if Spock stayed dead, however? I like Spock and Leonard Nimoy is fantastic but what if, right? James Kirk and company were already pretty wrecked and emotional about the situation, so it would've allowed them to build off of that pain and further explore the crew's grief.
It would've set up some pretty exciting concepts in films three through six. Some of which, like The Undiscovered Country could remain the same, while others like The Final Frontier would be radically different and hopefully for the better. It would also allow for Saavik to have a larger role in the final four films, filling in for Spock as the new Vulcan.
It may not have made things better, admittedly, but it'd be interesting to see if the film series is more even with such a major change to the core of the roster.