The most popular Community program managed by the DriveThruRPG/Onebookshelf team is probably the DM’s Guild. At the DM’s Guild marketplace authors and illustrators can create and sell D&D 5e content, legally and keep a chunk of the money from those sales.
Pelgrane Press has announced and opened the Gumshoe Community. In particular, some Ashen Stars content is now open to the community of authors and creators.
There are examples of third-party content for Ashen Stars already, and they’re good guides to the sort of content that Pelgrane welcomes. There’s Hidden Depths from Jakob Schmidt, Alexandria-Class Vessels Deckplans by Ralf Schemmann and Rogues’ Galaxy by Chris Sellers.
You can read more about the Community Content programs and discover other publishers who offer them at DTRPG.
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