In Generic text spans [foo] with special cases I entertained the idea of special-casing – by current CM spec terms: – link targets to support more of HTML inline elements. To get an actual <a> link, the first parameter within parentheses () of inline links or after the colon : in reference link definitions would then need to be enclosed in angular brackets <> to be considered an URL and therefore trigger link output behavior.
For local anchored links to ids, this would look like [text](<#ID>) → <a href="#ID">text</a>. This enables repurposing [term](#ID) for definitions, turning it into <dfn id="ID">term</dfn>. The ID would be optional: [term](#) → <dfn>term</dfn>.
As oh so often, life came in the way and I didn’t pursue this project any further.
Earlier, in Em/strong vs. i/b or cite/dfn etc, I considered to treat asterisk * and underscore _ emphasis distinctly and also special-case quadruple repetitions thereof. This could be used for other semantic elements or for presentational ones (by classic HTML terms), e.g.:
*em*,**strong**,****i****,********b********,
_cite_,__dfn__, …*em*,**strong**,****dfn****,
_i_,__b__,____u____
In the same post, I also suggested that Mediawiki syntax could be a precedent. In Wikipedia, defined terms, by convention, are put between triple apostrophes on both sides: '''dfn''', although WP does not generate <dfn> but <strong> elements from that; and <em> from double apostrophes. I would couple it with 'q' quotes and ''cite'' reference titles.
PS: The requirement for angular brackets <> would not need to be as strong as worded above. A slash / or well-known scheme identifier followed by a colon : should also suffice.