Star Trek is a major force in the world of science fiction and why wouldn't they be? They've either innovated many concepts that sci-fi properties embrace in the 21st century or helped popularize them. One of the unique things that the brand does that's pretty synonymous with science fiction troupes includes the retcon.
Be it through time travel or alien gibberish, Star Trek has always found ways to avoid dramatically changing things permanently. Maybe that's for the best, as the formula that Star Trek follows for their shows and films works. Essentially, you find a group of characters fans will pay to see (or watch week in and week out) and you throw everything you can think of at them.
What you don't do is permanently remove them from the equation. Nor, do you change up a show's concept so much that it changes the essence of what it is. Fans want excitement but they want only so much. What they really want is consistency. So sometimes Star Trek does things that could be interesting as heck, but they opt not to in order to maintain the status quo.
What would happen though if Star Trek were bold enough to make these huge changes? What if certain characters died for good, came back for good, or had an entire storyline retained? Well, that's kind of what we're looking at today. We're looking at four times that the Star Trek franchise could've had a major shift to their entire essence and how it would've gone.