The Star Wars films are part of one of the most beloved franchises in modern cinematic history. The franchise has created eleven major movies, several spinoff TV shows, books and comics and countless references in other popular media. However, if George Lucas had gone with his original idea for Return of the Jedi, things would've gone in a very different direction.
Of course, at the end of Return of the Jedi, Luke Skywalker has just defeated his father, Darth Vader, and stands above him as the half-built second Death Star begins to crumble around them. But while the final movie ended with Luke accepting his dying father and forgiving him, the original ending was a lot darker.
During the film’s conception, the writing team of Lucas and co-writer Lawrence Kasdan wanted to give the film a darker, more mature tone. In fact, early in the process, Kasdan even convinced Lucas to change the title of the movie from Return of the Jedi to "Revenge of the Jedi" because he thought that “return” was a weaker word.
The original ending of the movie would've had Luke taking Vader’s helmet and placing it on his own head, claiming, "Now I am Vader.” He would then reveal his own plan of eradicating the Rebel fleet in order to rule the galaxy with his own hands, essentially following in his father’s footsteps. This ending would have shaped the future of Star Wars in unimaginable ways had it happened on screen.
This darker version of the script also lends itself to a much bleaker outlook on the universe of Star Wars. In fact, another idea Lucas and Kasdan originally had was causing Han Solo to die midway through the film, meaning there would have been one less hero in the fight against the Dark Side. Perhaps, in this new reality, the Sequel Trilogy would've had a new protagonist in the form of Luke's sister, Leia Organa, but it's hard to say how different things would be had such a pivotal character been killed off decades before the new set of films even went into development.
In some ways, these stories were explored in the Extended Universe that Disney now calls Star Wars Legends. One of the best examples of a "Dark Side Luke Skywalker" is told in the series Dark Empire, written by Tom Veitch. This series features Luke working for a revived Emperor Palpatine and symbolically taking the mantle of his father as the Emperor’s new apprentice as he tries to gain control of the galaxy.
As far as Star Wars cinema goes, though, some of the plot points explored in early Star Wars drafts or in the Legends universe were used in the new trilogy. Ben Solo fits the role of good Jedi gone Sith and kills his own father. However, for the original films to go this dark right at the end of the trilogy may have been too much for its young audience. Lucas must've felt this way as well, as ultimately he decided to change “Revenge” back to “Return” and rewrote the ending into a much more positive one that ended the series on a much happier note. And it seems, given the success of the franchise, that Lucas made the right choice no matter how interesting the alternative might've been.