Not to malign DITA in any way, but Markdown is, by nature, HTML, isn't it? I'd like to have a "syntax switcher" button that let me convert Markdown to HTML and vice-versa. (Just as I would like this for RELAX NG compact syntax <-> XML syntax; sure, I can use trang as an external tool with generate/convert schema, but it would be nice if I could just be looking at a schema, click a button, and see it in compact syntax, eh?)
(resent to include the list address) What will be the support you will expect apart from knowing the extension and having a new file template? Both of these I think can be configured right now.
I was thinking more along what Tom mentioned in his reply, to define a MarkdownML language and provide visual editing, specific actions and conversion to Markdown. Of course this can be done also as a DITA specialization or we can have MarkdownML to DITA transformations as well.