What's something you've heard almost got made or almost was changed about a canon before it was released where you want to look into the parallel universe where it happened to see what it looked like
I think the one I most wish had happened at the time, though in retrospect I wouldn't have wanted the actress to have more prominence given how she's turned out, would be for Kirstie Alley to return as Saavik in Star Trek VI instead of Kim Catrall playing a new character as more or less her weird knockoff
It would have had so much more weight if the character stabbing them in the back had been the returning one from the high point of the series, not a new one no one cared about who made it seem like Spock was setting himself up as a skeevy mentor to promising young Vulcan women
Other options: what if Tom Paris on Voyager had been replaced by the very similar character played by the same actor who set Wesley up to lie about getting another cadet killed in that one TNG episode; what if Ro Laren had come back as the first officer on either DS9 or Voyager; what if Voyager had gotten a consultant on Native issues who wasn't a fraud
The Dark is Rising film rights were originally picked up by Jim Henson Pictures. I would have loved to see that movie rather than the watered down version we actually got.
I don't know any specifics, but my own impression and the consensus I saw from others was that the movie felt like it had been severely edited at a late stage to up the proportion of action and cut out a lot of the talking and politics
just looking more like the concept art