So you think
!foo[](){}is preferable to just!foo[]{}?
Not hugely important to me. I’m fine if the core language directives use some special syntax; I don’t think we really need to “explain” ref links ([foo][ref]) in this syntax.
On the other hand, consistency is nice - the two existing forms that use have a shared syntax for the () part: url, and possibly string for title. We could enforce this consistency: allow () on custom directives, but force it to have that same syntax. Not all custom directives need a url, for those that do, having them all use the same basic syntax (rather than having to remember which key this or that particular custom directive uses) is good. (I liked the examples of !video[foo](/foo.mpeg)/etc, those seemed very easy to understand if you already knew how images worked.)
Using () on a directive that doesn’t use that information is the same as passing a key it doesn’t use - it’s just wasted data.
(Things with more complex needs, like the slideshow example from previous that needs multiple urls, couldn’t use () for that. That’s fine, I think.
Edit: I expressed an opinion on the syntax inside of {} here, but I was wrong. Matching PanDoc and MarkdownExtra is valuable here; we should use their syntax, per http://talk.commonmark.org/t/consistent-attribute-syntax/272.