Another idea just because I want to spit radical ideas tonight:
This would have to be done in a redubbed fanedit, and would depend heavily on how Episode IX turns out on how usable this idea is, even with a redub.
Darth Plagueis, Palpatine’s former master, was not fully destroyed. He cheated death, and 26 years after his apprentice’s death, has resurfaced.
Working off of his apprentices Empire, he founds the Ultimate Order. The master of life and death, he reanimates the ancient Sith Masters to serve him: Darth Revan, Darth Bane*, Darth Malak*, Darth Zannah*, and even his traitorous apprentice, Darth Sidious. However, they’re only shells of their former selves, slaves to Darth Plagueis.
His might has brought the fledgeling Republic, still struggling with remnants of Sidious’s Empire, to its knees, and Luke’s new Jedi Order was not able to adapt to this challenge. The risen Sith killed all his Jedi, including Han and Leia’s only child Ben, and forced him into hiding on the first Jedi Temple of Ahch-To, where he has spent the past 6 years.
However, in these 6 years, the Ultimate Order has completely conquered the galaxy. However, Leia and Han’s experience in the Rebellion have allowed them to create an even more effective Resistance against Plagueis’s Empire.
In this time of need, however, Leia sends Poe Dameron on a mission to an old ally, who has a map to the first temple. She needs Luke to come back now.
*Knights of Ren?
Not sure where it would go from there. Kylo/Revan killing Snoke/Plagueis, though, would be an ultimate act of self-determination and might be paired with a return to the light?