> Is there a usecase for allowing a macro instead of a procedure-call as
> the first element?

As in defining a nested macro instead of nested procedures? Interesting
question.

I'm not sure, and the syntax would probably be hard to figure out. A
procedure call is a flat list so if there are nested lists in the
argument list of a procedure definition, we know they have to do
something special.

By contrast, a macro such as syntax-rules can destructure its argument
list as a tree, so if there are nested lists in a macro definition, the
assumption is that it means de-structuring nested lists from the macro call.

(Procedure call argument lists could destructure trees as well, but
since procedure calls evaluate their arguments, the user would have to
quote those trees when passing them in, making them less convenient.)