FWIW, there's a #pandoc IRC channel on Freenode (created way back in 2012).  It has ~ 50 participants, but isn't very active.  It can be accessed via the Web using either:

 * Kiwi (an IRC client): https://webchat.freenode.net/#pandoc

 * Element (a Matrix client): https://app.element.io/#/room/#freenode_#pandoc:matrix.org

Perhaps, as a start, that channel could be used by those who want a more synchronous discussions.  While IRC doesn't support markdown, if you use Element (the Matrix client) to access that channel, that client supports basic markdown formatting.[1]

 [1]: https://element.io/blog/riot-web-1-5/