That's kinda what alists and plists do! They mix up the abstract "map" or "table" data structure (something that maps keys to values) with some concrete implementation of it. '((key . value) (key . value)) and '(key value key value) are literal representations of two particular map-like data structures, whereas {key: value} or {key = value} could have any concrete representation that the language implementer desires.
Maybe I should have dropped the "hash" from "hash table" in my above posts, so that it was clear that I was interested in the syntactic benefit of using one syntax for mapping names to values, and not some imagined efficiency gains from having (read keyword-function-call) include a hash table instead of an alist.
(defun foo (&key foo) ...)
Whereas having a syntax for hash table {} and using this syntax in definitions/calls clearly indicates a conflict. What is it? Coincidence? Purpose?
There is nothing to be gained by hashtables.
The Common Lisp keyword syntax actually is a bit more powerful then what your PCL example shows:
(defun foo (&key ((var keyword) init var-arg-supplied-p) ...)
Generally I think it is a slight mistake to use specific data types in arglists - basically mixing syntax and data types.