Cyan released a teaser page for a new game called 'Firmament'. Details remain sparse at the moment with regards to its content, and currently, based on what we've heard from Cyan, it doesn't seem probable that it'll be completed or released anytime soon.
Firmament Game - Teaser Website
UPDATE - Cyan has a KICKSTARTER active to complete Firmament, and needs your help to get the game made! The campaign ends on April 26th, 2019, with a goal of releasing the finished game in July 2020!
UPDATE 2 - KICKSTARTER FOR FIRMAMENT SUCCESSFUL! The game is funded, and it'll likely be released this year - 2020. Expect more news to begin appearing here as release approaches.
I backed the KS myself with - among other things - a sale of a copy of 'From Myst to Riven' and a batch of handmade artworks as supplemental bonus items. I'm hoping that Cyan pulls off something amazing here that surprises us all with how good it is.
Here's a 15-second video ad teaser, unofficial, that I've made relating to the Kickstarter: