CLOS classes that can be constructed by calling them

callable-class.lisp

(defclass callable-class (standard-class function)
  ()
  (:metaclass sb-mop:funcallable-standard-class))

(defmethod sb-mop:validate-superclass ((class callable-class) superclass)
  (or (typep superclass 'callable-class)
      (typep superclass 'standard-class)))

(defmethod initialize-instance :after ((class callable-class) &rest rest &key &allow-other-keys)
  (declare (ignore rest))
  (sb-mop:set-funcallable-instance-function
   class
   #'(lambda (&rest args) (apply #'make-instance class args)))
  (setf (symbol-function (class-name class)) class))

(defclass foo ()
  ((x :initarg :x :accessor foo-x))
  (:metaclass callable-class))

(defclass bar (foo)
  ((y :initarg :y :accessor bar-y))
  (:metaclass callable-class))