I remember reading about it online and even seeing some rough drafts, and watching the Justice League Snyder Cut brought this back into my mind, and I hope to one day make a radical fanedit of RoTJ that’s a “Kurtz Cut” (It would be called Revenge of the Jedi, its original title). Here are some of the changes:

Two Death Stars under construction
A key setting is the Imperial city world of Had Abbadon (would later become Coruscant). Had Abbadon’s moon is known as Endor, and it has a shield generator on it that protects Had Abbadon from invasion.
Wookies live on Endor, and have been imprisoned and enslaved by the Empire to build settlements on Endor for residents of the overpopulated Had Abbadon.
There are two sail barges over the Sarlaac pit, and Leia never goes to Jabba’s palace. She’s headed to Endor from the beginning.
The final space battle takes place over Had Abbadon, and Lando is in command of a Mon Cal cruiser instead of piloting the Falcon. The two Death Stars aren’t destroyed by fighters flying into them, but by blasts from Rebel capital ships.
The Emperor’s throne room is in an underground chamber full of lava.
The Wookies are freed and help the Rebels fight stormtroopers on Endor.
Han dies in Leia’s arms after being shot in the final raid on the Imperial shield generator base on Endor.
The Force ghosts of Obi-Wan and Yoda appear to protect Luke from the Emperor’s force lightning.
Darth Vader kills the Emperor by grabbing him and diving into one of the pools of lava.