This is just part of commonmark - CommonMark Spec
A link contains link text (the visible text), a link destination (the URI that is the link destination), and optionally a link title
There are a whole bunch of other things you can do within the title section in regards to text and formatting but nested links are banned.
Reference links are a useful feature I don't see talked about too often and I think these could be a prime candidate for abuse.
You essentially have three parts, link text (like a normal link), link label (which links to somewhere else) and a link label reference which contains your actual location.
e.g.
[link text][link label]
lorem ipsum...
[link label]::/eaa0ac795a954df09982c1a96c760b28
I can imagine that it would be relatively easy to use that link label to contain metadata for categories without breaking anything. For example this works:
[this book][Type:book, Title:Cats]
...
[Type:book, Title:Cats]:http://cat-book.com
Otherwise if these aren't sufficient then at the risk of creating yet another standard (obligatory xkcd), with the prevalence of markdown based or markdown-supported note taking applications it almost seems sensible to have a new "markdown-notes" specification that would allow portability of markdown based notes to different applications that can leverage common features.
The issue of portability is something though - Joplin has a bit of advantage storing notes in the database where anything Joplin specific can be 'trimmed' when exported to pure markdown (like the internal ids do). Apps relying on .md files don't really have that option. It also means that some degree of conversion would need to happen when opening notes in external editors.
There are a number of interesting solutions to the problem - there was a markdown editor I found particularly interesting with its implementation of text and image manipulation.
To sum up, no idea, I've been randomly pecking away at this reply in between pointless messages from work and lost track of what I was trying to say more than a few times. It really is a case of either trying to work around the existing spec or making your notes technically incompatible with actual markdown which reduces portability.