The use-case you used is the one I explicitly said is in-keeping with the spirit of Markdown.
For example, I like extending
![]()to mean “embed this resource” not “create an image tag”. It is keeping with its semantic use and the spirit of MD.
A theoretical !youtube[Funny video](http://youtu.be/dQw4w9WgXcQ) would render as “!youtubeFunny video” which, IMO, is a perfectly acceptable fallback since it still provides a link to the content and the exposed exclamatory declaration isn’t so jarring as to render the output confusing or meaningless.
But to specifically answer your straw man, no, I do not see any problem with this either:
```youtube
http://youtu.be/dQw4w9WgXcQ
```
It would render as an embed where a “youtube” plugin is available and as
http://youtu.be/dQw4w9WgXcQ
where no such plugin is available, it’s a reasonable fallback and non-breaking to existing implementations.
But even that doesn’t answer a multitude of other questions:
- Why would you resolve this to a
<figure>element? - Why an iframe instead of an object or video embed?
- What if someone else wanted a different output format? Are there now multiple different potential outputs for “!youtube” depending on what site you’re posting on or what parser you’re using?