This information has the ability to potentially ruin elements of the plot of any currently released book for the reader. It should also be considered uncanonical and will potentially completely change in the future.
The Sixth of the Dusk sequel is an unpublished cosmere novella by Brandon Sanderson set on First of the Sun.
Brandon initially stated that he has no plans to write a sequel to Sixth of the Dusk, but later came up with an idea he liked that involved Dusk going into the the Cognitive Realm and wrote an outline and the first chapter for that story. He is hesitant about writing more and releasing the novella, however, because it would have extensive spoilers for the space age of the cosmere. At the virtual release party for Rhythm of War, he read an excerpt from what he had already written; a transcript of the reading can be found here.
In the draft read at the Rhythm of War release party, the story takes place about six years after the events of Sixth of the Dusk. The Ones Above are starting to grow impatient and want to build an installation to breed and collect Aviar on one of First of the Sun's islands. Vathi and Dusk are trying to figure out how to delay their progress and are also dealing with people from another world, likely Roshar. Dusk realizes that the people of First of the Sun are hopelessly outmatched and decides to go through Patji's Eye, the perpendicularity on the island Patji, to try to enlist help.