"Extension developers should open an account on Wikimedia's Phabricator, and request a new project for the extension. This provides a public venue where users can submit issues and suggestions, and you can collaborate with users and other developers to triage bugs and plan features of your extension." - https://mediawiki.org/wiki/Manual:Developing_extensions#Providing_support_/_collaboration
I began writing an extension to implement PhotoSwipe JavaScript gallery to be implementable in my MediaWiki website. It is a work in progress that I would like to share the code base with others so that it can be improved more than just my own coding abilities. Also I'm still fairly knew to developing with MediaWiki, so there definitely will be coding issues that can be improved or fixed. I don't know what these are yet as I am still learning.