Your comment is making a lot of assumptions about how licensing agreements between the publisher and BGA work. Generally, the publishers have a lot more control over whether and how their content appears on BGA than you're assuming. Here is a link to the template license that BGA uses with publishers:
https://en.doc.boardgamearena.com/image ... Pub_en.pdf
In practice, a game can only progress through Alpha, Beta, and Stable release on BGA with the explicit written approval of the publisher who has rights over the content. The decision to modify the BGA implementation of a game in order to include official expansion content, fan-made expansion content (regardless of who has rights to that content specifically), or house rules ultimately lies with the publisher.
For Wingspan specifically, the digital rights are held by Monster Couch, the developers of the official app. The only reason it is allowed on BGA is because Monster Couch and Stonemaier games have agreed to allow it. As the developer of the implementation, I am not privy to the exact publisher agreement used and how it handles BGA revenue, but this blog post and Jamey's contemporary live streams give more details on this arrangement:
https://stonemaiergames.com/stonemaier- ... an-on-bga/
As much as I would love to implement new content, Stonemaier has been consistent in their position that the BGA implementation will only include the base game.
That's not even to mention the rights of the creators of the fan-made content. Even if Stonemaier and Monster Couch agreed, the creators of the content would need to license it with BGA. Posting content themselves on BGG for free for users to print at home and play, allowing Stonemaier games to post it on their website, and implementing it on BGA are all very different things in terms of intellectual property. The creators of this content have chosen to share their work with other players and have certainly agreed to be featured on Stonemaier's site, but that doesn't mean they would automatically agree to that content being used for the direct financial benefit of BGA, Monster Couch, and Stonemaier games.
