Using > will create <blockquote> elements that are appropriate for quotes.
As far as other elements, there are a number of other proposed extensions.
This one covers attributes:
This one covers a generic block syntax
This one covers generic plugins
This one covers <aside>
None are official yet. They’re all at the discussion point, and there are other proposals. I recommend reading through everything that’s been proposed to see if your syntax fits in with one of these previously discussed styles. It also wouldn’t hurt to add your voice to the conversation, especially if you’re an implementer.
History is made by those who show up
In this case, the spec is really made by those who implement it. That’s why there’s so much credence given to existing implementations (such as testing via Babelmark).
That’s not to say you should run off and cowboy code your way to an inconsistent implementation, but trialing one of the generic syntaxes would help as a test case for what works and what doesn’t.