Following Rob's advice "If it doesn't make it to the mailing list it didn't happen."
After a chat with Markus Glaser, Mark Hersberger, and Jared Zimmerman at the Architecture Summit, we have decided to create a wiki page to define the specifications of this project. Your ideas and feedback are welcome:
https://www.mediawiki.org/wiki/MediaWiki/Gallery_of_extensions
https://bugzilla.wikimedia.org/show_bug.cgi?id=46704
While we have no idea where the resources to develop this project will come from, we agree about its importance, and we think that defining a plan is the obvious first step. Markus & Mark will lead this discussion as part of their efforts improving MediaWiki as a product for third parties.