One idea (I’m not sure if this is what you were thinking) would be to make the Knights of Ren similar to the Imperial Knights from the Legacy Era: Force users who reject the Dark Side, but serve an authoritarian regime, perhaps out of some sense of a “greater good.” That way, the story won’t require bringing the Dark Side back after RotJ, but will portray a whole new, different corruption of the Jedi ideology that has to be defeated. I wouldn’t have them massacre Luke’s Jedi Order, but I would have them convert most of Luke’s students away from his teachings, leaving Luke disheartened but not totally broken.
I love this idea. It allows us to revive the conflict between Empire and Republic, but in a more refreshing and innovative way. I have a few ideas based on this.
This version of the Trilogy begin with the conquest of Coruscant by the New Empire and the fall of the New Republic, with the Imperial Knights settling in the Jedi Temple on Coruscant. They convert most of the Jedi to their cause and exile those who don’t want to follow their ideology. So, Luke and a small group of exiled Jedi move to Tython along with the leaders of the New Republic Who were exiled by the New Empire. After that, they all start a resistance to defeat the New Empire and restore the Jedi Order and the New Republic. However, unlike the Sequel Trilogy we got, in this Trilogy Luke’s Jedi Order would be similar to its Legends version, so now the Jedi allow marriage and attachment. And Han and Leia have only one son, that is, Anakin Solo, who joined the Imperial Knights and that at the end of the trilogy returns to be a Jedi.
Furthermore, it would be very interesting if the figure of Anakin Skywalker had a central role within such a Sequel Trilogy. I mean, Luke was the one who redeemed him and made him fulfill the Prophecy, so it would be nice if Luke was constantly trying to remind everyone of the good side of his father. It would be nice if Luke discovered all the good deeds Anakin did before he became Darth Vader, and if he tried to teach the Jedi to respect the figure of Anakin Skywalker, as the Chosen One and as the Clone Wars hero he was, instead of constantly continuing to condemn what he did when he was Darth Vader.