###!tl;dr: Warning/Recommendations to help guard or clarify ambiguous inputs or bad practices. Presented via stderr or inline html, when a flag is set.
During authoring (like when you are typing up a post in discourse forum software), I think it is a good idea to implement a warning/recommendation system (much like what you see when compiling C.
The philosophy of markdown is that all inputs are valid inputs, so error system is out. However once we make CommonMark larger and more extendible (via generic directives), we really do need to implement a warning system to encourage good authorship.
Another use for this system, is dealing with ambiguous inputs. If there is an ambiguous input, the parser can pick the most likely option and throw a warning on it. This would give the author a chance to check if the parsed output is correct.
!( example.com/image.png ) would throw a recommendation to define an alt text descriptor which is important for blind people.
This means users are encouraged to use
!( example.com/image.png) to explicitly leave
 blank. Which would serve as a small reminder to define it. But at the same time, still allowing for lazy typing.
Presentation: This can be via typical warning logs ( via stderr ), but I would recommend that the warning be inline html like this <!> where you can click to expand the warning information. And only shown via a warning flag in the parser, that is recommended to be activated when the author is viewing/editing it, but not for the readers.