If there's one thing in all of history I could have changed, it would be seeing Sylvester Stallone replace Arnold Schwarzenegger in Terminator 2: Judgement Day.
28 years later, my prayers have finally been answered. Thanks to Ctrl Shift Face on YouTube, AI learning has been put to its full glory and Stallone's face has been digitally imposed over Arnold's in the classic bar scene from the movie.
In all seriousness though, I feel like in the near future we will start seeing more movies starring younger versions of actors like they just did with Samuel L. Jackson in Captain Marvel and I guess they kinda pulled it off for a few seconds in Terminator: Genisys. The possibilities for sequels and prequels could be limitless once this technology is perfected and streamlined.