Hi Karl and Timothy, thank you Karl for reviving this important topic.
I think our collective priority should be to work on https://orgmode.org/worg/dev/org-synxtax.html so that it reflects the current Org syntax. Hopefully we can do this before Org 9.6. As discussed with TEC, we can factor out suggestions from this document so that it is not a mix of facts and hypotheses. Then we can work on suggestions for evolutions of the current Org-mode syntax chunk by chunk, as a long-term goal for stabilizing changes for Org 10 (2023 ?) What occurred to me while rereading this thread is that definining a syntax for a IETF RFC on an Org mimetype probably needs to be done not just by this Emacs Org-mode community, but by bringing together other "consumers" of .org files, from ecosystems outside of Emacs. Such a collective work could lead to define what subset of the Org syntax is useful as the corner-stone for .org files everywhere - which is what you rightfully brought up with "Orgdown". If successful, such a process could end up in defining the minimal and official "Org syntax" while allowing implementations (like the one for Emacs org-mode) to supercharge this syntax if deemed useful. Perhaps TEC is right and we will end up having the minimal syntax being the one we currently use for Org-mode: we'll see. But we need volunteers: one to work on worg/dev/org-synxtax.org (I'm assuming TEC can lead the work here) and one to set up a discussion with people implementing Org in various places (you ?). I suggest to take this sequentially and not tackle the second work before we're done with the first one. 2 cts, -- Bastien