Cyan released a teaser page for a new game called 'Firmament'. Details are 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!
Currently, this KS is a little bit above $860,000, with a goal of $1,285,000, and under 10 days left - so they're going to need a fresh wave of enthusiasm to carry this over the finish line.
To that end, in an effort to raise funds for the KS, I'm selling off a batch of my Myst stuff, from vintage, limited-run books to handmade art. More info on that, and a display of my deranged obsessive enthusiasm, is being posted on the Firmament Kickstarter page.
Here's a 15-second video ad teaser, unofficial, that I've made relating to the Kickstarter: