Have there been any discussion of extending markdown with support for link relation types?
That would allow you to give meaning to the links. Which in turn can allow HTML representations and a user agent to do things like expose navigation for links with relation type next, prev, up, etc. Or possibly things like import of another markdown file into the current one. It could also use any link with relation stylesheet to be included as a style sheet in the resulting HTML.
An early suggestion would be syntax like this:
[link](/url "title" "stylesheet")
or
[Link](http://a.com)[rel=stylesheet]
It could also support future possible link params, for instance:
[Link](http://a.com)[rel=stylesheet][target=_blank]
I suggested to treat everything after the space-terminated link destination and before the closing parentheses (or end of line in reference link definitions) as an info string, usually starting with a title enclosed in quotation marks or parentheses. Within such an info string, there can be various conventions specific to certain implementations or output formats. Link relations in HTML could be one of those. The exact syntax would be out of score for Commonmark.