Thanks for your input.
I don’t think at this point this is necessary. Obsidian is (or tries to be) commonmark compliant and uses some commonmark extensions.
There are no modifications but 3 additions:
Wilikilinks [[]], block references ^ and comments %%.
I stress the difference between modifications and additions because you are not forced to use the additions.
I’ll move this thread to meta and we can use this thread and the help vault “how to format your notes” as a specification. I don’t expect major deviations to happen.
A page in the hub is also a good idea. Feel free to open a pull request.
Regarding plugins, I don’t think we want to bind plugin developers to adhere to any format.