Ryan Warzecha posted an announcement this afternoon in the Cyan Discord:
[2:09 PM] GreyDragon | Cyan:Intangible AssetsWe are happy to announce that the MOULa Intangible assets are being released to the public. Lore on these “Unexplored branches” will be rolled out at https://guildofarchivists.org/. If you want to know more about the development of these spaces, check out http://openuru.org/index.php and https://www.guildofwriters.org/.
"Intangible assets" refers to a swathe of material that was planned for Myst Online but never completed or released. This includes everything from partially-built world models to concept sketches and scribbled planning documents. Some of these ideas were retooled for Myst 5, but by no means all. (In fact, some of it was retooled for different Myst Online Ages that did get released.)
This material is not brand-new. Several years ago, a group of fans started working with Cyan to update Myst Online. The company handed this Intangibles archive over to them for that purpose. Unfortunately, the project petered out after a couple of years. (You can see a remnant web page of the original Intangibles project. It was active from 2014 to 2016 or so.)
Cyan has now decided to release the entire archive under a Creative Commons license (CC-BY-NA-SA 4.0). The beans actually spilled last night when a few people noticed this pull request on the H'Uru (open-source Uru) Github repo. It has not yet been merged, but you can browse the files in Hoikas's repo or download the full package.
(Needless to say: spoilers.)
The Guild of Archivists site is now being updated with (in-character) information about the new Ages.
What happens next? The fan community is of course actively discussing this question. Everyone wants to see the material added to Myst Online, in whatever form it can be added. But this obviously depends on volunteer effort. The timeline for MOUL updates has always been "It happens when it happens." In the meantime, enjoy the concept art.
(Speaking of which, did you know that a new small Age was added just last month? You can follow my spoilery update log...)
Here's an unrelated Myst fan(?) project:
To all fans of the original MYST... I am remaking my original Selenetic age in Unreal 5 using the few things I’ve learned as an artist over the past 28 years since it was first released. I’ll post up progress and images here and on my Instagram page. There will be a free playable[--@ChuckCarterART, May 28]
Chuck Carter was one of the lead artists on the original Myst. He hasn't been part of Cyan for a long time, but he worked with Cyan to publish his original game Zed a few years ago. Now he's going back to the original design document to create... what would you call it? Not exactly a reboot of Selenitic, and you can't say it's a fan project when the original artist weighs in.
In case you don't remember (hah), Selenitic is the Myst Age with audio puzzles and the Dreaded Subway Maze. Chuck says he wants to make the island larger and add more stuff to discover. Follow the twitter thread for more info. Should be fun.