(defclass mc (standard-class)
  ((foo :initarg :foo :accessor foo :initform 'some-default)))

(defmethod sb-mop:validate-superclass ((class mc) (superclass standard-object))
  t)

(defmethod shared-initialize ((o mc) slots &rest args &key foo &allow-other-keys)
  (apply #'call-next-method o slots
         :foo (etypecase foo
                (null foo)
                ((cons symbol null) (first foo)))
         args))

(defclass example-with-foo () ()
  (:metaclass mc)
  (:foo wow))

(defclass example-without-foo () ()
  (:metaclass mc))

(print (list (foo (find-class 'example-with-foo))
             (foo (find-class 'example-without-foo))))

(defclass bad () ()
  (:metaclass mc)
  (:foo bar baz)) ; should signal an error